Specifically a database that shows a JPEG of the front cover with a UI that allows you to filter by date, and/or author. Click on the JPEG to get a short summary. There are so many questions from people trying to identify books from fragments of memory (myself included). We humans are "visual" in many ways pertaining to memory. I'm thinking that if one could just scroll through a set of book cover images and adjust a few simple filters - it would be a useful app/website. It would have to allow users to add more book entries. Does such a site exist? Maybe I should build one...
The isfdb.org website is a useful resource I've used several times.
Isn't that one of the main things that Goodreads has? Perhaps not quite the way that you are envisioning, but since they've got a pretty good head start, it seems unlikely that you would be able to get any market traction unless you have some truly compelling features that they don't.