Audible-and-Kindle Cross-Format Library Bridge That Lets You Own a Book Once and Read or Listen Anywhere Without Repurchasing
Amazon's Audible (audiobook) and Kindle (ebook) libraries are siloed. Whispersync only works for the specific subset of titles where the user already owns both formats, which means most readers who want to switch between reading and listening end up paying twice. With Pocket dead and Mozilla's read-later experiment over, and Audible's biggest gripe being lock-in, the opening is a third-party library bridge that uses TTS (or licensed studio audio) to play the user's Kindle library, and a paired ebook reader for their Audible library, gating around DRM with the user's own credentials and never touching pirated content.
Read the actual Amazon TOS before naming the product. The straightforward play is to start with EPUB libraries from Apple Books, Kobo, and direct Google Books purchases (all formats you can legally read with user credentials) and offer high-quality voice synthesis. Add Audible playback through the public Audible Cloud Player. That covers 60-70% of the audience without picking a fight with Amazon's KFX DRM. The legal mess on the Kindle side is what scares off bigger players; building a clean non-DRM-cracking product is the entry.
landscape (3 existing solutions)
Cross-format reading is the most-requested missing feature in the Audible/Kindle reviews. Amazon won't ship it; Amazon's competitors can't because they don't have the libraries. There is a permissioned-API niche if Amazon ever publishes one, and a legally-grey gap until then.