Are there any rules, guidelines or suggestions as to what the best use of books are? By books I mean the ones that give a character a +1 boost to a single stat.

I found a +1 Charisma book, and I'm contemplating whether I should use it on my main character, who has terribly low charisma (so low that he really can't talk to anyone effectively), or on the character in my party with the highest charisma.

Depends on whether you consider the game a party game or a solo game where only the main character does matter.

Most players choose the second and give all the books to the character they created, the other option will yield a benefit on the other hand.

You already know the facts you need for your decision, do as you wish, the game is beatable either way. There are even players who finished the game without finding the book.

I always dump them on my main character, since that way the bonus will stay with you forever. If you use it on others, you'll lose that +x when you start BG2, since their character files aren't imported :) (unless you're using BGT, I suppose (never tried it)).

SPOILER
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
That being said, charisma specifically isn't all that necessary in BG2, since you'll find a ring that sets it to 18 pretty early in the game.

Yeah, that is one of the major things that BGT does than no other mods does: allow you to keep party stats (without using a trainer, obviously) between games. However, NPC stats are not consistent between vanilla BG and vanilla BG2. IIRC, some NPC stats are raised for BG2 in rather significant ways (and none are lowered). BGT allows the player to choose which set of stats to use for each of the affected NPCS, unless I am mistaken.

However, I always just give the books to CHARNAME, even if they are next to useless for him/her (such as +1 INT to a character 19 INT, or +1 CHA to a character with 10 CHA). I am just too much of a min/max power-gamer not to :P.

Yeah, I do the same thing. I choose to think of it as my character 'maturing' as the series goes on. :)

I'm planning to give the +1 strength book to imoen, damn she's a weakling, must have spaghetti for muscles

I'd suggest give the tomes to people who need them mostly. e.g. +1 STR tome to your fighter and +1 WIS to your cleric/paladin; +1 CHR to your leader as he/she is the usual one to talk with shopkeepers. But again, assuming you are going to export the characters to BG2, it's not a bad idea to give all tomes to the main character.

Unless you play BGT, the only character that takes the extra points with him to BG2 is your main character, that makes him the first choice for me to give the books to.

Mind however, that there are a lot of instances in which the extra points are as good as useless on a certain character, even your main one, and can be very benificial on your other character. You can find out for yourself what a 1 point increase by consulting the tables in the manual.

If for example my PC is a Fighter with 18(50) STR and 10 INT and I got Imoen in my party (STR 9, INT 17), these are the effects of the STR and INT tomes respectively:

Giving Imoen the STR increase as Aliasalpha would have it, only serves to increase her weight allowance by 20 lbs (from 50 to 70), but she wouldn't be your packhorse anyway. Giving the STR tome to a Fighter with 18(50) STR increases his STR to 19, meaning: an extra +2 to hit, +4 damage and +280 lbs weight allowance (from 220 to 500). If my main character would't have a fighter class, the benefits to the group (and so to your PC as well) would be much greater by giving the STR tome to Minsc, Kivan or Shar-Teel: the 18+ STR warriors.

Take a look at the INT tome: giving it to my Fighter with 10 INT, increases it to 11, which doesn't do anything (besides *SLIGHT SPOILER WARNING* a slight advantage when dealing with Intelligence-devouring creatures in BG2).

If (what's often done, and even canonical, she starts in BG2 that way) Imoen is dual-classed to Mage, giving her the INT tome would increase her INT to 18, resulting in a better chance to learn spells and making her able to have 4 more spells/level in her spellbook. When I have Imoen as main mage (same holds true for Dynaheir and Xan) it would mean giving her increased flexibility in spellcasting and almost doing away with the frustration of having to erase lesser useful spell if I find a useful spell for her but the spellbook for that level is full.

So if a certain increase is next to useless for your main character, even the PC himself benifits from giving it to the character for whom that stat is the most important one - by increasing the group efficiency and thus the ease with which xp is earned and items are gathered.

This however is totally worth it because you don't want your ~200 hitpoint fighter to die after a few hits because his intelligence dropped to zero.

Yeah, but I seem to recall that said monsters drain 3 intelligence per hit, so you die in four hits regardless.

Oh yeah I think you're right there, but just for the sake of argument, raising it from 9 to 10 would go a long way. Also helps you out of mazes faster.

If you want to let your mage learn more spells per level or have a better chance to learn spells, you could also give him/her a potion that increases INT. There are several different potions that raise INT in both games.