Here's something handy for any of the folks out there with Nexus products that have already updated to Android 6.0/Marshmallow. It will also be just as useful for anyone who wants to get the jump on understanding Marshmallow before it finally comes to their device. Google created a quick start guide for their new mobile OS.
Google's official quick start guide for Android 6.0 Marshmallow is 62 pages long and includes details about nearly every new feature. This includes how to use Google Now on Tap, heterogeneous storage, Doze, App Standby and more. The guide even demonstrates tricks you can use on your Android which have been around for quite some time. Overall there is definitely a great deal to learn from the guide.
While some might find it a bit TL;DNR, it's still probably worth flipping through to find some of the useful tidbits. Here's a link to the Google Play Store to grab it: Android Quick Start Guide, Android 6.0 (Marshmallow) - Books on Google Play