This FAQ is specific to Z-Wave. There are many free or low cost references for zigbee available–just stick to the ones for the zigbee Home Automation 1.2 profile in order to understand use with SmartThings.
The Official Sigma Designs Training Materials
As far as zwave goes, the following answer is unpopular but true: zwave is a proprietary standard owned by one company, Sigma Designs. Buy a developer’s license for $3,000 and you’ll get access to lots of training materials, none of which can be published openly on the Internet or bought as standalone books.
Open Z-Wave: Good for Learning Z-Wave Concepts
There is an open zwave project which is reverse engineering and is good for learning concepts, particularly the management of sleepy devices, the practical uses of associations, the concept of the basic command set, and some stuff like that. It will certainly help get you out of the mindset of WiFi networks. But it’s an abstraction, not firmware details.
Information for the General Public from the Z-wave Alliance
If you want a general public information overview, check the Z-Wave Alliance site. It’s also a good place to read about coming changes and new devices. You can also look up the official conformance statements for individual devices. But you won’t find technical command references there.
Z-wave for Raspberry Pi Projects
If you want to add zwave capabilities to a Raspberry Pi, zwave.me has a nice add on board, fully z-wave certified, available for many different countries. Called the RaZberry. Cost is about $75, and is available from most retailers that sell Z-wave products. Reading about their projects can be educational even if you never build one yourself.
An Introductory Z-wave Textbook Anyone Can Buy
There is one approved Z-wave textbook that doesn’t require a Dev license and Is used in some college network engineering classes, but the english translation has horrible spelling errors (“sealing” instead of “ceiling.”). Also, although the title says " basics," that’s in the context of electrical engineering, not DIY. A typical passage: “In a conductive load the capacitive reactance exceeds the inductive reactance. Hence the load draws a leading current.”
Mesh Topologies in General
As far as mesh networking in general, I’d recommend looking for specific home automation references. Otherwise you’re going to get deep into electrical engineering as a prerequisite for most of the texts, and that’s likely nowhere near the stuff you’re actually interested in.