It leans very heavily on the fact that LLMs have very broad latent knowledge of.. well, most books written in English, really. So it tries to do a good job of finding you books which aren't just another one by the same author, or happen to be in vogue right now.
You can try it out a little without signing up, import your reading history from other services (Goodreads, storygraph, particularly nicely with hardcover), or just take a pic of your bookshelf. If you do sign up, it'll get better at suggesting you things as you talk to it more about things you've read (hopefully it does a reasonable job of holding an intelligent conversation on that score), and you can also then connect it to claude and friends via an MCP or play around with the API.