Found myself wondering about this after finishing 'Mage's Book.' The narrative leaves the protagonist on a cliffhanger, definitely setting the stage for more. However, as far as I've been able to dig up, there isn't a direct sequel novel that continues that specific story. The author seems to have moved on to other standalone projects in a similar magical vein, but the threads left dangling in 'Mage's Book' remain untied. It's a bit frustrating for those of us who got attached.
That said, the author's other work, 'The Alchemist's Ledger,' explores a neighboring kingdom mentioned briefly in the original book. It's not a sequel, but it feels like it shares a universe—some of the magical systems overlap, and there's a passing reference to the events of 'Mage's Book' that made me perk up. It scratches a similar itch, even if it doesn't answer the big questions. I ended up enjoying it on its own terms, though part of me still hopes the author circles back someday.
For now, I'd say check out the author's other titles for a similar vibe, but don't expect a continuation of the same plot.