1.) Books are just for lore - you've done the one that involved a quest so I think there are no more like that - sell them or dump them (they're not worth much - I tend to just read them and then put them back).

2.)You can store things in any container, provided the area doesn't change - I don't think that's an issue in BG1 but strongholds in BG2 change from before and after you get them.
So find a place you go to often, like the Friendly Arm Inn, and put them in a container there.

3.) Yes, unless there's an area change - like maybe in the Copper Coronet in BG2. Can't think of a BG1 example, except a modded BG1 if you have a certain NPC, the FAI changes it's store.

4.) Shop prices depend on charisma and reputation -> 20 rep will make things cheaper to buy (not sure about selling) - but generally, the only equipment worth lugging back to sell is magical stuff (+1, +2, "of the xxx" etc) and gems. [Edit:] and Jewelery items like gold necklaces/rings/etc - (thanks to Pi4t for the correction). Scrolls and potions are better off being used.

5.) Bless is very useful, as is "protection from fear" (mage) or "remove fear" (cleric) (which acts as a protection too). Fear is something that affects the party a lot in BG1 and 2.
Protection from petrification (mage) is only useful against a certain enemy in BG1 but there's an area where it's essential (though potions of mirrored eyes do the same).
When you get 'haste', it's really handy.
Web can work well, like entangle though, it's too easy to get your party.
Sleep works wonders against mobs in BG1 (less useful in BG2) - I think it affects your party too but I can't be sure - fire it ahead of your characters though before you charge ;)
Edit: Oh, and you can use invisibility on your thief (Imoen?) and she'll be able to detect and remove traps while scouting ahead, avoiding detection and clearing the way.

It's been a few years since I played, so it's easily possible that I superimposed NWN's rules over BG's. I can't 100% remember. You may very well be right.

Minor correction to extend that list to jewellery. This goes doubly in BG2 (or BGT with a certain tweak enabled) where you can get magical gem bags which compress a lot of jewellery/gems into one item slot. The only downside to jewellery compared to gems is that they don't stack with each other. The gem bags I mentioned have a limit by total number of items, rather than stacks, so they neatly get around that issue (and mean that if your bag gets full, it's often a good idea to remove all instances of a gem you have a lot of, and carry it in one slot, freeing up room in the bag for the gems you have smaller number of and jewellery). In BG1, you're unlikely to get enough enchanted rings and amulets for everyone anyway, so you can treat the free slots as extra inventory space for unenchanted versions of that item. Unenchanted gear is never cursed, you'll be pleased to learn.

sorry, yes (I was thinking of that when I said 'gems' but got the wrong word lol) (I'll edit it into the list above for togetherness).

nice sharing!!

I'll add that once you get to BG2 you may want to hang on to some of the most valuable gems you find (diamonds etc.) as they may have uses other than selling.