Below are several links I have found useful.
Active Directory Maximum Limits – Scalability
Post-Graduate AD Studies (Numerous links to advanced Active Directory topics)
MCM: Core Active Directory Internals (Deep dive into Active Directory structure that helped me understand the databases a bit better)
Test Lab / Self Training While I would really prefer to spend a little money and buy a TechNet subscription which I did for years Microsoft has decided that will no longer be an option. Here are a few poor substitutes for creating a test lab or getting free Microsoft evals.
TechNet Eval Center – Get them while they are current MSFT will remove these when newer version of software come out.
Microsoft has a number of prebuilt scenarios that include a lab manual as well as a block of time to try the new techniques in the cloud. This is also a pretty impressive example of Hyper-V automation.
If you want a more free form lab experience holSystems is the company that hosts Microsoft’s test labs.
Microsoft Virtual Academy provides a number of online training scenarios.
Scripting with Windows PowerShell Technet resources for PowerShell
Learn Windows PowerShell 3 in a Month of Lunches by Don Jones – This is the book I always recommend when people ask me for a PowerShell learning recommendation.