[Adoby]

Most likely some of the books in your collection already have good metadata and nice covers.

I would suggest that you do this in two steps.

Start by only adding epub and mobi books. They are likely to have good metadata and covers embedded. When adding the books you can have calibre use the embedded metadata and ignore the filenames.

If the titles and the authors are correct it should be easy to download extra metadata like series information and descriptions of the books.

This could quickly and with a relatively small effort give you a very nice library to start with. Stop there and try to make it perfect.

Make sure everything is just as you would like it. Series information. Cover sizes. No duplicates. Consistent author names. Correct sorting, just as you prefer it. Good genre system. ConsisRatingstent tagging system. Links to authors. Information about books read/unread. Conversions to correct formats for your reading device. Integration with your reading device and reading apps. Add news downloads. Setup a backup system to protect your investment in time and effort. Find complementing metadata sources. Virtual libraries. And so on.

Enjoy your perfect calibre library and read some books.

After that start adding and converting the rest of the books when you are in the mood. Preferably do it in small batches you can fully complete in a few hours, and preferably books that you are more likely to read soon. Perhaps books that can fill gaps in series you already have in the library, or by authors you enjoy especially. Or interesting topics. Or books that you have heard others talk about. Most likely you will not live long enough to be able to add and fix all your books. Perhaps some books are not worth the effort, and can be deleted? Take some time off now and then to read a book or two.

Also consider adding books from other better sources than your own.

If you try to add junk you're likely to end up with a junk library that is no fun.