The Big Book of Logic Puzzles - Sujikai 400 Hard (Volume 20)

Sujikai (literally "diagonal") is a variation of Sudoku. The puzzle consists of a triangular grid of cells containing digits from 1 to 9. The objective is to fill a grid with digits so that each cell contains a digit and no digit is repeated in any column, row and diagonal in any direction. Also, no digit occurs twice in any of the three larger 3 x 3 square regions and any of the three larger triangular regions enclosed by thick borders.