My library system has a program we call Libraries Unshelved. Donated books in good condition have labels placed with our library's information and explaining that whomever has the book is welcome to either keep it, pass it on, or return it to the library. We also include bookmarks or flyers with program or digital services information.
A program like this could be expanded to provide passive readers advisory by changing the information on one of the labels or flyers to include readalikes for the book. This will help guide the reader to other titles if they enjoy this book while also providing a clue as to whom the book will appeal. Don't forget to include non-fiction and youth materials in your program. Non-fiction and fiction can also be used as readalikes for each other. There are many non-fiction titles that are compelling and readable as any fiction book.
If most of your donations go to a book sale, you may need to work iwth your administration or friends of the library group to get a program like this started. It could be expanded to include patron input if you supply a form that allows a patron to include a short review and their own readalikes. That information can be included with the book as well.
Work with your local businesses, recreation department, senior center, and other buildings where the public gathers to identify places where you can place your own Unshelved collection, Places like laundromats and barbershops can be great places to start. Some restaurants and coffee shops may also want to participate.
Your library's presence in your community can be expanded by placing these curated books in public places outside the walls of your library. Including readers advisory information will help place your library as where readers can come for assistance and support while also helping to create what readers advisory expert Becky Spratford calls "a community of readers" with your library at the center.