Is Harry Turtledove? Compare the similarity in output to Patterson. I just noticed that Glenn Reynolds received a copy of Turtledove’s second book in a quasi-alternate history series he’s been working on. Turtledove shines when he’s applying what he knows. Even if the Videssos cycle wasn’t artful prose, the books had plots which moved along, while his historical, Justinian, had incredible source material to work with. But a lot of his more recent production just crawls along, rather like the later books in The Wheel of Time. Additionally, I have to say that some of it definitely has an almost Terry Brooks sensibility of mixing & matching plot and character modules instead of crafting the narratives in a more organic fashion.
Note: Piers Anthony could arguably be the James Patterson of science fiction as well, but he seems to have turned himself into a totally marginal figure for those beyond the age of 11 at this point.