The only way I've found to do it is to do it via ISBNs..
1. Export an xls file of the Librarything library.
2. Open it in Libreoffice, with the ONLY the option "Tab" selected under the
Separated By import options (you can't have commas selected, because they'll ruin the formatting by trying to read your tags as different headings etc...)
3. Scroll rightward, to the ISBN column. (It's column "H" on my export file.) Click the column heading letter to select the whole column.
4. CTRL + C to copy THE WHOLE THING.
5. Paste them into a text editor.
6. Use the Find + Replace option to get rid of ISBN-less entries, by replacing all
\n[]
with nothing. Then get rid of all the [ and ] using the same method.
7. Also delete the
ISBNs heading at the top (which it would also have copied.)
8. Bring up Calibre! you can run multiple libraries for it, so it helps to create a new library for the project you're managing; it makes things less confusing. Click on the Calibre icon within the program icon bar, and go to
Switch/Create Library. Make a new folder for the library with whatever name you like and then select to "Create an Empty Library at the New Location", using the new folder.
9. Now go to Calibre > Add Books > Add from ISBN. Paste the ISBNs into the Calibre dialogue (one number per line!), set any tags you want to add to all of them and then click Okay. You'll see a dialogue come up with options to download metadata, download covers, download both or configure downloads, to fill in the missing data. Calibre will fetch the missing metadata, including Title, Author, Publisher, Publishing Date, Description and some tags (such as General, Fiction) and append it to the book. It does not create/download an ebook; you'll just get a nice empty file you can use like a library card catalogue.
(I'm still trying to figure out how to import my custom ratings, shelves and tags from Librarything- and also, some books don't have ISBNs due to age or rarity, so it can be a problem... Some books might have to be added in manually. But for most recently-published/popular books, the method I just listed should work.)