this is the first book of the series. it is the standard. killer sudoku is a combination of Sudoku and Kakuro. it has the rules of both. not only do all the rows, columns and subsquares have all the digits, but the cells are grouped such that each group has a sum. has become my favorite puzzle. although the book is normal paperpack size, the puzzles are as big as the larger selling competition versions. no waste of small and simple puzzles, it is a book filled with brown and black belt level. highly recommended - the whole series. best of genre