When I was in University I always used Let's Go and occasionally Lonely Planet.
Let's Go is now very poorly researched and very inaccurate so I've dropped it.
I have occasionally used Frommer's and find there can be useful information in it. Haven't used it in a while though.

My friend swears by Rick Steve's but I don't find it has anything that isn't in Lonely Planet.
I still like Rough Guide as a backup.
Basically, though, I just use Lonely Planet.
I would never take a library copy on a trip. In fact I rarely take a full copy of any travel guide. I photocopy the sections I need and leave them behind as I move on.