Something funny about Carr is that for all his professed and undoubtedly sincere devotion to the detective story, the authors he’s most reminiscent of are either outside the genre or at its periphery.
Granted, Carr owes a lot to Chesterton and Doyle, a debt he never tried to conceal, but what he seems to have loved about them and tried to emulate is less their crime fiction than their approach to writing – Chesterton’s emphasis on imagination over realism and colourful prose on one hand, and Doyle’s efficient storytelling on the other – Carr’s plotting techniques being quite different from theirs or from any other contemporary mystery writer for that matter.
Perhaps however the two writers who made the most decisive impact upon his writing are M.R. James and Stevenson, and he never hid that either. His penchant for the baroque and the supernatural certainly come from the former whereas he inherited his literary doctrine (for he has one, which is understudied in my opinion) from the latter. Stevenson thought that adventure was a noble subject, perhaps the noblest of the all, needing no philosophical or psychological excuse, and that places were as crucial to fiction as characters. That and him also being a biting critic of realism, which couldn’t but appeal to Carr.
Ultimately of course Carr was his own man and can’t be reduced down to an amalgamation of previous writers, but it’s I think worth pointing out that only his love for the genre made him a mystery writer as he might have developed into something else altogether; Devil Kinsmere/Most Secret may be the closest we’ll ever get to that Carr That Never Was. Whether we should regret or celebrate it is of course entirely up to you fans and readers.