This places it among some fairly rocky precedents, from J. I. M. Stewart's caricature of JRRT as 'J. B. Timbermill' in his Oxford quintet (e.g., A MEMORIAL SERVICE ) through James Owens' bizarre take on the Inklings in THE SEARCH FOR THE RED DRAGON  (in which Tolkien is a linguistic dunce who tries to hide the fact he can't read anything but modern English).*
Somewhat less egregiously, there's been the wish-fulfillment of Xian fiction by Jeschke (e.g., EXPECTATIONS ), and the graphic novel about Charles Wms, HEAVEN'S WAR by Micah Harris , which is rather interesting, despite the author's near-total ignorance about JRRT. In a somewhat different category is Rbt Velarde's CONVERSATIONS WITH C. S. LEWIS, which isn't quite a novel; I've only skimmed Velarde's book, but I suspect it might turn out to be the pick of the lot.
Here's hoping that Downing's can rise above the (low) standard set by his precursors in the field of Inklings-as-Fictional-Characters*
P.S.: One odd thing I noticed; as soon as I ordered this, Amazon started lobbing recommendations for random Catholic books at me. So far as I can tell, there's nothing particularly Catholic about Downing's book; I guess we'll see.
*shades of the bottom-of-the-barrel-scraping of Th. Wheeler's THE ARCANUM -- one of those books which features characters based on real-life people (H.P. Lovecraft, Conan Doyle, Houdini) whose fictional counterparts turn out to be nothing like the real peope whose names they've been given.