Harry Potter and the Cursed Child, by John Tiffany and Jack Thorne
Some Harry Potter fans are bound to be disappointed with Harry Potter and the Cursed Child, but I enjoyed it. Written as a play script, the book has an entirely different feel about it.
But I enjoyed the story of Scorpius (Malfoy's son) and Albus' friendship, and I like time travel. So it worked for me. Not anywhere near as good as the actual books, but an entertaining spinoff.