I would imagine the only reason the 2 volume checklists and 1 volume versions of Handbook of Bird and Handbook of Mammals can exist is that the art was created for the original books. The earlier books effectively helped pay for these volumes, not to mention gave them time for all the illustrations to be created. A reptile/amphibian series might be a hard sell as a multiple book series, but I think it would be an even harder sell for a single or two volumes.I have long desired such a work for reptiles and amphibians. I have previously bought 15 out of the 16 original volumes of the HBW series, and own five of the mammals series. However, after purchasing All the Birds of the World and now the single-volume mammal equivalent, I really would most prefer such a volume for reptiles and amphibians. Or actually, one volume for reptiles, and one for amphibians (since collectively there are so many species). Now that I own the single volume work for the birds I probably would be better off selling my fifteen volumes of HBW. Obviously there is a tremendous amount of information in the original work that is completely absent in the single volume tome, but for my uses the original work is overkill. How many of you would prefer an "All the Reptiles/Amphbians of the World" over a ten-plus volume handbook series?
Although releasing books like the illustrated checklist and such probably further decreases the odds of any new series being made. At least some potential set of buyers will not bother getting individual volumes, since they expect the omnibus volume to come out.