The Maul minifigures is inspired by the character’s appearance after his restoration in The Clone Wars, perhaps specifically his look from Solo: A Star Wars Story. It ties in nicely with the character’s imminent return in the new episodes of Star Wars: The Clone Wars.
Here is the book’s official synopsis:
DK’s LEGO Star Wars Character Encyclopedia: New Edition is updated to include all the latest and new LEGO Star Wars minifigures! Meet amazing minifigures from all the Star Wars movies- including new minifigures from Star Wars: The Rise Of Skywalker. Discover incredible LEGO Star Wars facts, such as which minifigure is the rarest and which can be found in the most LEGO sets. This new updated edition contains more than 200 best-loved minifigures, including Luke Skywalker, C-3PO, Chewbacca, Rey, Kylo Ren, and BB-8. The book also comes with an exclusive LEGO Star Wars minifigure!