Remember, this question and its answer are specific to our cataloguing on LibraryThing. Here, I get most of my listings from Amazon in the US. Occasionally I've resorted to LC, but sadly, it's been rare that something not listed in Amazon shows up in LC. In 2 cases, I even had an LC call number in the book but could find no record. The other sources I've used occasionally are Israel Union List (for various religious or Hebrew language books), National Library of Australia (for some needlework books and pamphlets which were printed in Australia or New Zealand and which were catalogued there but not here), and Amazon.fr (for the occasional French volume, obviously).
The only things that I don't find in the systems and have to hand enter are very old books (I do have a handful of books printed before say 1950) and some religious books (because apparently a few of the Jewish publishing houses are not distributed through Amazon, don't register copies with LC, and haven't made it to the Israel Union List especially if they're actually published here). This last exception is getting to be less of an issue as recently it seems a few of the larger Jewish publishers (this is relative, I'm still specifically referring to publishers whose only audience is the US Orthodox population, a tiny number of people relative to the reading world) are newly distributing through Amazon, and books which in the past did not show up in their system do now.
I will be interested to see if others have specific sources with benefits or subject areas I don't know about.