Here is the great Aerohive High Density whitepaper: http://www.aerohive.com/pdfs/Aerohive-Whitepaper-Hi-Density%20Principles.pdf
And here is the page with other Aerohive whitepapers as well: http://www.aerohive.com/resource-library
Just click on Whitepaper under "Types"
I know this one is really good too:
"I'm Going To Let My Chauffeur Answer That"
Math and Physics for the 802.11 Wireless LAN Engineer
as it says "A Discussion of What Every LAN Engineer Should Know About 802.11" ;)
These videos are too! http://www.wlanpros.com/wlan-pros-summit-2014-videos/