Shiva has published a book by Ian Stewart and Robin Jones, Computer Puzzles - For Spectrum and ZX81.
The authors pose a series of problems, some old, some new, for which they have written programs to demonstrate the problems and to enable the user to try solutions quickly. The programs are interesting but when you solve one of them you have the same feeling you have when you have beaten someone in a game.
Also included is a Spectrum section for those lucky enough to have the machine. The price of the book is £2.50.