When I first wanted to learn about the ins and outs of Wi-Fi many years ago I discovered what I consider the bible on the topic, '802.11 Wireless Networks - The Definite Guide' by Matthew Gast. It has since held a prime spot in my bookshelf but obviously it is now a bit outdated as it doesn't include information on 802.11n and the current latest and greatest 802.11ac standard. So when I recently wanted to deep dive into 11ac I had a look around on the web and discovered Matthew's '802.11ac survival guide'. After reading it I can very much recommend it because the level of detail and writing style was just what I've been looking for.
While it can be read on it's own if you have some background on how Wi-Fi works, newbies are better advised to first read the general chapters of his original book and then jump into the 11ac survival guide. Both books and his 802.11n survival guide are available as ebooks. If you buy directly from O'Reilly you can download it it in various formats and all of them are DRM free. I'm glad I looked before I bought the DRM protected Kindle edition over at Amazon.
To get a first impression I can also recommend this video on Youtube where he gives a talk about 11ac.