There is only one ending.

How you complete certain quests may open or close other quest possibilities, but this will not cause problems later. You might miss out on the 'best' solution or certain side quests, but there is no punishment built into the game for doing the wrong thing.

Books provide background information and sometimes hints on things to do related to a nearby area, quest, item, etc. For the most part once you have read them they can be sold or destroyed.

You can eventually bring body parts a necromancer to have him create a creature for you, which can provide support, or at least a distraction, during battle. Save the best versions of each type (though you will likely get better parts when you are closer to obtaining your creature anyway), and sell the rest.

Some people like the ghost for healing, but I think the demon is better for combat (have not tried either). I think there is a 'master summoner' spell which should allow more than one summon at a time, but I think it didn't in the unpatched game, at least.

After about half way through the game you will have the ability to re-assign all your skill points, for a price (IIRC 5k gold the first time, and it doubles each time after).

The statue you need to find is in the same dungeon with the 3 amulets, in the central area.
it is in a room at the top of the stairs going down towards the south (IIRC) branch, fairly well lit, with a plant on either side, and a grate / metal bars forming one wall of the room.

I maxed Lockpick; there is lots of loot in locked chests, but you could probably get by without it.

You will meet Bellagar the mage several more times, but you will eventually get some loot. There are chests in at least a couple of those circular shrine areas.