I found two online books called "Games of no Chance" and "More Games of no Chance" in the following URLs:
http://www.msri.org/publications/books/Book29/contents.html
http://www.msri.org/publications/books/Book42/contents.html
They are collections of articles about mathematical combinatorial games or "games not involving chance or hidden information." Most of the articles are fairly theoretical and academic, and I wish I had the time and knowledge to read and apply all of them. You might find some interesting ideas to include into your games.
For me, the most approachable articles were "The Angel Problem" by John Conway; "Coin Moving Puzzles" by Demaine, Demaine and Verrill; and "Sowing Games" by Erickson.
