Now that Spotify have added a bunch of Audiobooks and Podcasts, I think it would be a nice idea to have the option to: Pause your audiobook/podcast > go back to listening to music > and then be able to get back to your book/podcast and pick up where you left of, without having to memorize the time that you paused.
We're setting this one to implemented. Although there's not a specific 'bookmarking' feature, Spotify does now keep track of where you left off in a podcast, and store that progress for you across your devices and sessions.
If you'd like to see the same behaviour for audio books, we'd recommend making a new Community Idea for this. In the meantime, we've passed all of your feedback along to the right teams.