The following books by American Mathematical Society are very good.
1) Fixed points, by Yu.A.Shashkin
2) Stories about Maxima and Minima, By V.M.Tikhomirov
3) Intuitive Topology, By V.V.Prasolov
These books translated from Russian are very insightful and are not voluminous.
You can also look at the following combinatorial game theory book:
Surreal Numbers: How Two Ex-Students Turned on to Pure Mathematics and Found Total Happiness. By Donald Knuth.