Hallard T. Croft & Kenneth Falconer 
Unsolved Problems in Geometry [PDF ebook] 
Unsolved Problems in Intuitive Mathematics

Mathematicians and non-mathematicians alike have long been fascinated by geometrical problems, particularly those that are intuitive in the sense of being easy to state, perhaps with the aid of a simple diagram. Each section in the book describes a problem or a group of related problems. Usually the problems are capable of generalization of variation in many directions. The book can be appreciated at many levels and is intended for everyone from amateurs to research mathematicians.