If you log into the BookBeat app on your Apple Watch, you can stream books directly on your watch. The watch app shows a version of "My Books" that contains the last 25 books that you have listened to and that you still have an active bookmark in. The book you've listened to most recently is shown first.
Books that you have saved to "My Books" but not yet listened to are not shown in the watch app. If you want to a new book to the watch, you will have to start listening to it in the app on another device first. As soon as the book has been started, it will be added to BookBeat on your Apple Watch.