Always watch out for cyclists. Cambridge is very very flat, and they're everywhere. While these pedestrian only streets might seem real cool, they aren't really, and you get swarms of deadly cyclists hurtling down them, especially around the rush hour.
And if you're planning on cycling, watch out for the one way system. And the locals that yell at people who don't follow it.
The central library is indeed very important, but it's shut for renovations at the moment, due to not looking futuristic enough. The observant of such things may notice that Cambridge has rather more old buildings than usual. This is partly thanks to the lack of any bombs dropped on it in the second war. Apparently Hitler planned to make it the capital city after he took England, or something.
Which leads on nicely to the next point. Don't believe what the tour guides say, just appreciate it.