The Apple books App will certainly allow books in readable form (such as PDF or ePUB formats) to be imported to (i.e., be “shared” with) the Books App. Audio Books may need to be synchronised with iPad/iPhone using iTunes for Windows - using iTunes’ file transfer capabilities to import the Audio Book.
That said, I have never actually tried/needed to import an mp3 format file, Audio Book or otherwise, to the Books App. For mp3 files imported for playback on iOS/iPadOS devices, I have always used alternative players to the Apple-native Apps - as these are often considerably more flexible and/or have capabilities and functionality more appropriate for the media content.
As such, I might recommend that if you are experiencing any difficulties in using iTunes for Windows, to import mp3 Audio to Apple’s Books App, that you use one of the many third-party player Apps that you’ll find in the App Store. Alternatively, some “Googling” may pay dividends in either finding a step-by-step process to achieve successful import of mp3 Audio Books to the native Books App, or a more suitable alternative.
While not immediately resolving your difficulties, I hope this insight proves to be useful in guiding you to an acceptable solution.