Download AudioBookMark and set a bookmarks for your actual audiobook. The only thing, you have to do is to follow this four steps:
1. Open the Spotify app
2. Play an audiobook
3. Open AudioBookMark again
4. Confirm your selection
After that AudioBookMark saves the progress of your selected audiobook automatically and you are able to play them in the app – and the audiobook starts exactly at the point where you paused it, even if you heard music after that!