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.