I have just been thinking about an idea for a self-imposed rule that would reign in Bishops a bit while making them more fun early game:

A spell pick must be used on a spell that the character was high enough in level to learn at the time the spell pick was gained. In other words, no saving spell picks for higher level spells: At level 14, for example, you are only allowed to pick 1 6th level spell.

How does this rule sound? (Also, any ideas of what spells are best to pick? Remember that spellbooks are still allowed under this rule.)

Well, it is of course a thing you can do. It probably makes you want to focus on 2-3 spell books and share the load with 1-2 other casters (or hybrids). This also affects the spells you pick - with a Valkyrie you could let her do all the Divinity stuff (especially Magic Screen, Armorplate), otherwise those are important spells to pick rather soon.

But in case the Bishop is the only caster... I would do it somehow like this:

Creation: Mind Stab, Heal Wounds
Level 2: Sleep or Energy Blast
Level 3: Identify Item
Level 4: Enchanted Blade
Level 5: Armorplate
Level 6: Missile Shield
Level 7: Noxious Fumes or Fireball
---
Spellbooks found in Monstery: Light, Stamina, Bless, Divine Trap
Spellbooks found in Arnika: Cure Lesser Conditioin
Important Spells bought in Arnika: Magic Screen, Cure Poison, Cure Paralysis, Knock Knock, Insanity, Rest All
---
From then on it heavily depends on what you do next, which means it's not really possible to give good advice for the whole "middle game".
The most important spells are in my oppinion:
Level 8-10: Element Shield, Soul Shield, Cure Disease/Haste/X-Ray/Armormelt (most of those you can buy from Crock)
Level 11-13: Summon Elemental, Freeze All, Sane Mind (the Portal spells are found as books)
---
Then the important level 6 and 7 spells. My take would be:
Level 14-18: Resurrection, Quicksand, Boiling Blood, Might to Magic
Level 18-22: Tsunami, Death Wish, Death Cloud, Nuclear Blast, Concussion/Cerebral Hemorrhage (they do almost the same thing)

Here are a few thoughts that I have:

Mind Stab can be delayed to level 2. Instead, pick a spell at level 1 that has a Spellbook/Realm combination you want to level. For example, if you want to use Freeze Flesh, pick Frost so that you can start leveling the associated skills.

Identify Item can wait, as it can be bought in Arnika (and getting money is easy with Alchemy). Also, why Enchanted Blade (which can be bought in Arnika) rather than Missile Shield (which can't be bought until Trynton)? Also, Holy Water might be worth choosing simply because the spellbook will almost certainly not be found. (Why doesn't Lord Braffit sell that spellbook anyway?)

I find that Freeze Flesh is actually quite useful, and since I like to use an Alchemist in the party, that might take precedence over Noxious Fumes. Fireball is less useful because it only does damage. Whipping Rocks should be noted because it isn't buyable until level 11 for some reason.

Cure Disease can be replaced nicely with Restoration potions. (Cure Disease potions should either be sold or used to make Renewal potions, both options giving you more than enough money to buy a Restoration potion.) X-Ray is a good idea because it doesn't need higher power levels. Note that Crock doesn't sell 4th level spells until level 11.

For the 11-13 range, I would want Psionic Blast. It's a nice spell, capable of hitting enemies on the other side of a wall, and you can't get its spellbook until level 18. I'm thinking Heal All could be useful, but the Amulet of Healing can replace it. (I actually think the Amulet of Healing is overpowered with its +3 AC in addition to its other powers, and there are 2 that you can find, so you don't need to pay Crock 10000+ for them.) Also, why Sane Mind, which is a highly situational spell?

For 14-17: Resurrection can be replaced with items. Boiling Blood doesn't seem to be that great (and a Gadgeteer can get that spell in gadget form anyway). Then again, it feels like (excluding Quicksand) this spell level is less useful in general.

For 18-22: Why Tsunami, especially as the first pick? (Iceball is much better than low level Tsunami.) I would much rather have Restoration: At level 1 it reliably cures nearly all status ailments, and that spell isn't subject to enemy resistance. Also, why Nuclear Blast? It won't work on Rapax or while underwater. I would much rather have Earthquake, which works really well against flying enemies, especially when they surround you. (Also, if you have an Alchemist, Death Cloud doesn't stack, so Asphyxiate should be taken instead.)

Of note, my current ideas end up with no Fire spells picked for the Bishop, interestingly enough. Blinding Flash will be handled by my Alchemist, while Fire Ball/Bomb will be learned via spellbook.

By the way, the party i am thinking of is Valkyrie/Ranger/Bard/Gadgeteer/Alchemist/Bishop.

Many interesting ideas, and most of it comes to to how your party is constructed. Do you have a Bard and/or Gadgeteer? Then you won't need a lot of otherwise important spells.

I mentioned Tsunami (not necessarily as the first pick though) because its raw power is insane against the Rapax. Sure, at first you can only cast it at level 3 maybe, but even then it will do (w/o modifiers) 9-81 damage - Iceball does 14-70 damage at level 7. I know it's a cone spell, so you usually won't hit as many enemies, but this spell is insane.
I agree that Earthquake is probably better than Nuclear Blast, but I would rather spend my Earth points on Quicksand.

Again, everything depends on your party construction. If you are more melee based, you probably want to focus more on status effects. In ranged parties, damage spells are a bit better - even those which only work on 1 enemy. I really like Boiling Blood, Dehydrate, or Concussion/Cerebral Hemorrhage. Boiling Blood and Dehydrate do 14-266 damage at PL7, Concussion and Cerebral Hemorrhage even 14-336 (plus possible Insanity / Unconsciousness). In ranged parties this is great.

Btw, if your Bishop also covers Psionics, I really like Psionic Blast as a level 11-13 spell. It is the only spell that works on an enemy group, which is a great weapon to have (if you battle only 1 group or if you can only see 1 enemy of a larger group). It does decent damage (the same as Ego Whip, which is more than Fire Bomb or Iceball for example) and can cause insanity. One of the best midlevel offensive spells out there I think.

You are also right about Whipping Rocks, which is also a really effective spell especially during midgame..
Fwiw, almost all the offensive Priest spells are crap - they are all weaker than other spells if you compare damage to spell costs.

Thing is, a level 18 pure caster/Bishop (or a level 22 hybrid) can only reliably cast a 7th level spell at power level 1, regardless of skill. Hence, Tsunami (and other 7th level damage spells) is rather pointless at that level.

Earthquake, as I mentioned, is nice when lots of flying enemies are around. Quicksand feels like a bit of a waste in that situation, as you are using instant death against enemies that have low hp. (It's a shame there isn't a radius targeted earth damage spell at level 3 or 4; that would be ideal against flyers.)

Another alternative is Mind Flay; you end up with plenty of Mental spell points anyway, and it is worth noting that Element Shield doesn't affect it (but Soul Shield does). There are 2 major exceptions here, though both are special purpose spells:

Holy Water is basically Iceball with lower cost, worse range and only effective on certain creature types. Turn Undead is usually better (especially since it can paralyze), but there's no way for a character to produce the effect more than once per battle. Also, note that the Holy Water spellbook is extremely rare; it's a rare drop from 2 unique (non-respawning) creatures, and one of them is Lord Braffit.

Banish is stronger than the 7th level attack spells, making it the ideal attack spell to use against the enemies it works on. (Banish + Death Wish should handle every enemy with low Divine resistance nicely; there are no enemies that are immune to both.) It's not worth picking for a Bishop because spellbooks with this spell exist.

Yeah sure, but Banish isn't even Divinity-exclusive, and Holy Water is only useful for a couple of battles (mostly in Marten's Bluff) - after that you get Banish (which is way better) or have better things your Bishop can cast. And, as you said, you usually have to learn it, which - especially for a Bishop - is rarely worth it.

I have actually started this playthrough, and here are my spellpicks so far:

Level 1: Heal Wounds, Frost (to prepare for learning Freeze Flesh)
Level 2: Mind Stab
Level 3: Missile Shield
Level 4: Holy Water (I got to use this spell shortly after learning it)
Level 5: Freeze Flesh (had to do a little extra practice here)
Level 6: Noxious Fumes (I love this spell)
Level 7: Whipping Rocks (mainly because it can't be bought until level 11, and I didn't want to wait to get 30 Divinity)

My Bishop has just reached level 7. The spell pick for level 8 looks like it will be Armorplate unless I decide to go to the Umpani Base Camp early. Level 4 spells don't seem to have much to offer that's crucial early on. Note that she has 0 Earth Magic currently.

She has learned Enchanted Blade and Magic Screen from spellbooks, but doesn't have as much Divine Magic skill as in my previous playthroughs where I conserve spell picks.

By the way, I noticed a rather curious weapon called the Lance that Anton sells. It is a polearm that is usable by Lords and Valkyries (not Fighters for some reason), by every race except Faerie, and which has 4% Kill. Doesn't that weapon seem a little strange? (Why can't fighters use it? Why can it instant kill?)