Frost is a very solid writer. I haven't read any of them, but his first novel The List of Seven (1993) was mostly very well received. (The sequel slightly less so.) It's a piece of the kind of retro Victorian kitsch writing that only became fashionable later on, and seems like it shares some similarities with, say, The League of Extraordinary Gentlement (the comic, not the film).
The reason why people are Lynch fans and not Frost fans, despite the fact that Frost wrote most of what people enjoy about Twin Peaks, is (IMHO) that Frost is a better writer when he's wrestling with other people's ideas (or when he collaborates). It's telling that his best novel (The List of Seven) is highly unoriginal in its subject matter and bordering on fanfic (in a good way), and that his best work has been as collaborator with Stephen Bochco and David Milch as writer/story editor/director on Hill Street Blues and with Lynch, Peyton and Engels on Twin Peaks. His sense of dialogue and story structure is amazing, not least when he's editing other people's scripts or writing rough story outlines. His original ideas are often less interesting, unless sparked by something interesting and original. Which is why I think his Twin Peaks novel will be great! The characters are already there, and I'm sure David will pitch in with some story ideas since he's very much invested in the endgame of the novel (since I assume the new season will start off where the novel ends, more or less).
The other Twin Peaks books are uneven, but interesting. Jennifer Lynch (David's daughter) wrote the Secret Diary and Scott Frost (Mark's brother, also a screenwriter and later novelist) wrote the Autobiography. Both were rush jobs, the diary was released before the premiere of the second season and the Autobiography was released sometime late season two. It's rumoured that David Lynch had more input in the writing of the Diary than in the actual Twin Peaks scripts from late season one and early season two, and it's certainly darker and more impressionistic than the pedal-to-the-metal procedural thriller the series was at that point. The Autobiography is wildly uneven, but really funny at times. Also, the Windom Earle backstory is (in my recollection) better and more interesting than it was on the show - less cackling, more quiet menace.