Actually, the overdrive plugin does a pretty good job of covering gutenberg.
I've also added some gutenberg plugs in the wikidata-gui plugin, but wikidata's coverage of gutenberg lags at best and is incomplete typically. I considered fixing that and doing a mass import of gutenberg data into wikidata, but the wikidata people were too picky about how they wanted the data imported and I decided I didn't have time to follow their rules when they already don't follow them. (I kind of rely on that, their "ideal" data import is convoluted and would make it hard to match books if they actually used it.)
Wikidata does contain (spotty) references to many other databases (maybe a couple hundred actually), and the calibre wikidata-gui plugin (which I will update by the end of the weekend) can import any of them if you configure them. I've considered writing plugins to further use that imported data to get more complete data from the other databases than what is in wikidata. Ask nicely for specific ones and there's a slim chance I might be motivated.
But like I said, there's several hundred, and not all of them have API's.