A crowd-sourced global database of most field guides (printed, web and app-based) that can be filtered by location, taxa and other categories doesn’t exist. iNaturalist would be a good choice for such a database because of its main function is crowd-sourcing identifications on all taxa.
Here is a list of sources for field guides:
Wikipedia (https://en.wikipedia.org): great for background information on various taxa but poor on the fraction of field guides covered and is not filterable by location. Difficult to edit.
Book sellers (Amazon, Chapters, NHBS, etc): popular field guides included, with image covers and often previews. Obscure local field guides, out-of-print, and primary literature excluded. Not filterable by location, except for NHBS.
International Field Guides (https://bibleaves.library.illinois.edu/catalog?f[collection_facet]=fldg): A large database, over 6500 included, but database search is not that friendly.
Bugguide.net: Has a very good database of field guides for insects. Site is limited to USA and Canada. However it is filterable by taxonomy. Here is an example: https://bugguide.net/node/view/77/bgref?from=8. But you can’t filter to location, aside from the fact it is for USA and Canada. What is interesting about bugguide is the large amount of primary literature covered in its database. This extensive bibliography is part of the reason why bugguide has a higher accuracy rate for identifications.
The Ontario Field Naturalist’s Toolchest: http://www.ontariofieldnaturalists.ca/ This is my personal website of field guides. It is very comprehensive but still incomplete. My coverage of apps sucks because I don’t own a tablet or cell phone. My website is geared for Ontario and is divided into various taxa. It includes images of book covers, previews of books when I could find them, links to really old field guides on archived websites such as https://www.biodiversitylibrary.org/ when I can find them. I include local field guides that usually get overlooked like: Bats of Ontario (from the Matt Holder Environmental Research Fund). A useful field guide like this is easily overlooked by it is not listed on Amazon or found in a conventional book store. I also attempt to identify which field guides are the most useful, although sometimes I don’t get it right. This website is a pain to update so it often goes awhile not updated.
Implementation: To make it easier to implement for the programmers I recommend reusing user interface elements already implemented. For example, uploads of field guides could have a similar interface as uploading photos and browsing field guides could be accomplished something like the “Explore” feature with a few modifications.
Other filters to consider: Percentage of species coverage (all, most, common only), Life Stage (adults, immatures, fruits, signs etc), friendliness (layman, amateur naturalist, professional scientist), portability (foldout, compact field guide, desk reference), coverage (checklist only, identification guide, full life histories), usefulness (must have, useful secondary reference, limited usefulness)
Pros: Better identification from the community, more money in the pockets of field guide authors.
Cons: Great potential for SPAM, copyright violations, piss off bugguide.net with the duplication of their bibliography, no easy way to indicate what species of a taxonomic group are covered or not by a particular book (e.g. Newcomb’s Wildflower Guide)
This topic was originally intended for Feature Request, but I am moving it here because it represents a major functionality (composed of a lot of individual features combined) that might take years to implement if ever (so many other feature requests to take care of first) and likely should have a period of brainstorming.