Yes you and your spouse can share kindle books using different accounts

I don't know why this isn't made clear anywhere, and I believe this is fairly new, within the last several months. It apparently has to be done on a desktop browser through your Amazon account. Worked for me, I can now see all of my wife's books alongside mine on my Kindle for iOS, who has a separate account.

To enable a Kindle reading app to access shared Family Library content, visit Manage Your Content and Devices, select Your Devices, pick your device, and choose the “Show [name]'s content” option under Family Sharing corresponding to the other person's account. The books will then show up on your device once you sync it (i.e. via sync, or I'm sure, restarting the app).