Regarding spell books, it may be advantagous to keep them until you have enough books to fill up a spell. Each time you use a book, the spell level increases, but so does the level requirements for the next spell level.
If you're at level 12 and a the next level of a spell requires level 13 and you get a book, wait until you reach level 13, then purchase the skill, then use the spell book. You will raise spell levels a lot more quickly this way.