I’m also on 11.4. Here’s the precise steps I use that work:
1. Open Books and tap on Search.
2. Tap on Library in the upper part of the screen to the right of the Search bar under the Store, Window, and Help menus, not in the list down the left side.
3. Tap again in the Search bar and enter the partial title, and hit Enter. Depending on the search string, you may see results before hitting Enter.
Because Library search was already selected on my machine, it remembers the selection, so I can just type in the partial search phrase.