Good e-books on wifi
The IEEE 802.11 specification can be hard to read (since it is very detailed). If you are new to wifi or even if you want to deepen your expertise, here are a few books that will be useful for you.
Next generation wireless: 802.11ac
By no doubt, this is my favorite book on wifi. It is by far the most concise and detailed. The PHY section is very good as it the MAC section and advance features. The author made a good compromise of mixing lot's of details and basic communication theory.
802.11ac : a survival guide
This is another great resource for wifi. It is a bit more accessible than "Next Generation wireless LANs". It is geared toward users and network engineers with strong CS background (and less network background).
High performance browser networking
High performance browser networking
This book is more about networking than anything else. It has 1 chapter on Wifi and the book is available online.