You will be happy to know 3rd version of the book Mastering Proxmox is being worked at this moment. Plan is to get it published as close as possible to Proxmox 5 release.
Obviously I am a hardcore Proxmox user as well. Would not use any other hypervisor even if my life depended on it.
Thanks to Charles @greavette I found Nethserver. Although there are lot of room for improvement so I am happy so far. Have not used all features yet just email/webtop. I came from long years of ClearOS. Great product but due to the licensing change and the cost I was always on the look out for whats out there.
I do thank Nethserver @dev_team for the work they have done so far. Nethserver got great potential.