Extreme Programming Explained by @KentBeck is basically the bible for teams that pair program. Its reputation is well-deserved.

Even though helped popularize the idea of pairing, it actually only has two pages on the subject!

You'll probably be surprised by what it says: 1. Pair on all your production code, because
- it keeps you on task
- it encourages you to stick to team practices
- when you get frustrated or stuck, your pair can hop in and take a turn driving, leading to less frustration overall
- it helps come up with & clarify ideas