Reddit mentions of Applied Combinatorics

Since you're studying comp sci, combinatorics might be a really good topic--most of the difficulty students have studying algorithms comes from not having a strong enough background in combinatorics. It's also something that you can delve right into. I don't really have any book recommendations, but my undergrad class used Roberts and Tesman. I don't remember enough about this book to say much about it.

Alternatively, logic is super cool! A book on computability or mathematical logic would work. Herbert Enderton has written a book on each. I find his writing a little too informal, but they're good for getting a grip on the basic notions.