As I am planning all my different themes, I am trying really hard to find books to go with them. Literature is an excellent way to introduce new concepts, new vocabulary, and to explore things you otherwise couldn't. Erik is much more likely to understand what I'm telling him about if there's interesting pictures and an entertaining story line to follow.
The town we live in has a pretty impressive library (for a town of this size, anyways), mostly because they will bend over backwards to help you out. If they don't have the book you want (and let's face it, they're tiny so there's a good chance they don't) they will borrow it from another library for you. This means that as long as I plan far enough in advance I can get just about anything I want. Unfortunately for planning our Weather Theme, I didn't want to wait for books to come in, so I just crossed my fingers that they would have one or two books that would work and went for it. I ended up finding five books there that had something to do with weather (actually there were a few more, but they were too advanced for Erik) which was way better than I'd hoped and we've been enjoying reading them ever since.
In addition to the books we've been reading, I also found a free app for our Ipad that allows children to explore the weather. Erik loves to play with it and it's one that we'll probably keep around for a while.