The first is another Cadigan-penned book, and the official prequel novel, Iron City, which focuses on Dr. Ido, Hugo and Vector’s lives before Alita is revived in the film’s events.
The last book you should consider (besides the official art book, of course) is Dr. Ido’s Journal, which gives personal insight into the thoughts of Christoph Waltz’s character, as well as the world that he lives in.
However, two more books are available that run through materials that supplement that of Alita: Battle Angel’s story.