One thing the books often don't warn you about is the problems that can be caused by your own repair efforts. Some mistakes you make are minor. For example, if put your derailleur cable on too lose, your deraillerur may not work right, but you can go back and fix it. On the other hand, these sort of mistakes can be costly:
1. Setting the rear derailleur so that it throws the chain into the rear wheel.
2. Installing the bottom bracket improperly.
3. Installing the headset improperly.