Cozumel has a lot of bird variety and is a popular birding destination. I'd be surprised if nobody's made so much as a fold-out guide for it specifically.
And some guides do mention in their sale descriptions what regions they're for. Loads of guides online that say they're for part of the US, then in the description, say which states. Whether or not a guide has any mention of Cozumel endemics (particularly the subspecies, since there's only two extant endemic species) would be useful information to put in the sale description, since I'm sure some guides have left them out due to Cozumel being a separate landmass.
Do you know of a bird guide that, for birds like the Bananaquit and Yellow Warbler, mentions "this bird has a subspecies endemic to Cozumel"? Or one that explicitly contains Cozumel Vireo and Cozumel Emerald? I'm sure some guides do, and I'm curious which ones those are. That's all.