Books:
5 Regular Sixys
5 Wacky Sixys

Regular: the bold outlined areas are always 2×3. Wacky: the bold outlined areas have all sorts of shapes. Much harder!

All books are by creator Peter Ritmeester, whose first Sixy was published in The New York Times July 21, 2019.

Each books contains a foreword, a detailed How-To, and 200 puzzles with solutions.

For books in Dutch: www.pzzl.com