Peter M. Higgins 
Professor Higgins’s Problem Collection [PDF ebook] 

What can you do with your maths? You can use it to thoroughly understand all manner of things that cannot be dealt with in any other way. This book serves up a variety of problems and shows how mathematics answers them. Topics range from cracking codes to the persistence of recessive genes; from logic puzzles to classical geometry; and from planetary motion questions to predicting the market share of competing companies. And there are other problems where the mathematics itself is intrinsically surprising and interesting.