How far can I go with a Twilight themed cake without violating copyright laws? Can I put the book titles on the cakes?

My rule of thumb is that if you look at a cake and the first thing that comes into your mind is a copyrighted character/book/movie/etc., it's probably infringing (unless the creator got permission or used licensed cake toppers).

The word "Twilight" by itself in a generic font is not copyrightable, so you should be OK as long as you don't pair it with other Twilight cover elements or try to mimic the font, but the safest thing to do is use licensed toppers on a cake that fits in with the color scheme without duplicating any copyrighted elements.

I was considering the checkerboard, the chess piece, and the birthday girls name in the typface used on the books. Not sure if this is going past theme and itnto infringement.