I was there a few weeks ago and some things to consider:
- The majority of books are in French. There are 2-3 bays in English in which I found some gems. But lots is just not useful – even if you can read a bit. Great titles but stuff I'll never get to read cover to cover.
- The dollar is so poor that had I just ordered them off Amazon UK I would have saved a bundle. They were British imports so marked up accordingly. I bought'em anyways.
- When I went, the gal there didn't speak much English. Make sure you can ask how much things are (Combien est-ceci? is one way) as many books weren't priced.
Don't let me discourage you – it's worth seeing once. I personally wouldn't go again unless I'd just won the lottery. But there are some prints and that kind of thing too but they seemed overpriced given the weak dollar. There wasn't anything in English I couldn't get in the UK for cheaper.
I didn't find/see any hobby shops like you'd see in the US and even the bookstore at Invalides was kinda weak.
If you read French, disregard the above and enjoy! It's a bonanza for the fluent.
[edit .. just saw the OP is from UK ..don't know what the exchange rate is for UK'ers – maybe its a bargain for you]