I found a Burning Wall spell book in Mardaneous' house, which was the only way I could kill the spider at the bottom of the catacombs. As a warrior, I only used the spell another once or twice, with the heavy orcs when first leaving Aleroth.
You can make up for a lack of luck with persistence;
If you quick save before running your mouse over an item/chest/merchant or hitting the alt key, you can identify the item or trade with the merchant, then quick load to get different stats or inventory. This can be done repeatedly to get the best stats or specific bonuses on an item (save after finding good candidates) or looking for specific items (such as spell books or charms) from merchants. Don't waste too much time with this, though, it really isn't necessary to get the best possible equipment.
All/most bookcases have a small chance to contain a spell book. By saving and reloading, someone said they got something like 20 books just in Aleroth (in addition to the bookcases in town, there are at least 3 I recall not too far into the catacombs).
