How to Solve It: A New Aspect of Mathematical Method is a really good book. It was suggested to me by the best high school prof I ever had.
Other awesome books this teacher suggested were: Infinity Beyond the beyond the beyond, The Education of T.C. Mits and The Einstein Theory of Relativity: A Trip to the Fourth Dimension, all written by Lilian Lieber and illustrated by her husband Hugh Gray Lieber. These books were just reprinted recently go check them out.
Other awesome books this teacher suggested were: Infinity Beyond the beyond the beyond, The Education of T.C. Mits and The Einstein Theory of Relativity: A Trip to the Fourth Dimension, all written by Lilian Lieber and illustrated by her husband Hugh Gray Lieber. These books were just reprinted recently go check them out.