Reddit mentions of Principles of Helicopter Flight

"The Art and Science of Flying Helicopters"
http://www.amazon.com/Art-Science-Flying-Helicopters/dp/081382169X

"Principles of Helicopter Flight"
http://www.amazon.com/Principles-Helicopter-Flight-W-Wagtendonk/dp/1560276495

From the FAA site you'll find a wealth of aviation documentation including 'Rotorcraft Flying Handbook' located here:
http://www.faa.gov/library/manuals/aircraft/

All three are great books. ('Rotorcraft Flying Handbook' can be downloaded as a free .pdf book.)

If you're starting the training process, the first thing you should start doing is preparing for the knowledge exam, and the PPL test prep book (2020) is the best place to start. These are effectively the questions you'll see on the exam, if not THE questions. Most CFIs/pilots treat this as an SAT score, get >90% and you'll get taken more seriously. You'll need a FAR/AIM, which at first seems daunting but quickly becomes a bedside book. Jeppessen Private Pilot Manual is a good resource which consolidates a lot of the information nicely, and provides nice plots for things that otherwise feel a bit disjointed (e.g. medical certificate requirements, airspace).

Books that are helpful in terms of learning helicopter dynamics, my favorite by far is Principles of Helicopter Flight, as well as Cyclic and Collective. Many people like the Helicopter Flying Handbook, but as someone who knows better (I'm a PhD trained physicist), I found it to be garbage. The authors try to simplify things down to make it accessible to a lay person, but get a lot of things flat out wrong or are just plain sloppy with their descriptions. PoHF on the other hand isn't overly complicated, but provides a very clean and correct description of helicopter dynamcics. Don't be dazzled by the bright colors and pictures in HFH. Learning to Fly Helicopters is one I found to be a fun read, which lightly touches on flight details, but provides a lot of tangential tidbits around real world situations.

Study the Pilot Operating Handbook's for your training helicopter(s)! If you got some spare money, I'd recommend buying them (Robinson's can be found at their website, either free PDF or for purchase for ~$60). As you get closer to your check ride, you'll want to become more familiar with the Practical Test Standards which is the rubric the examiner will test you on, and there's several oral exam guide (general, helicopter) which are quite helpful.

I'd also recommend recording your rides and re-watching them. I did this and it was very helpful to go back and see what was happening, much like a quarterback watching film on Monday morning. You can find my PPL training videos on my YouTube channel, which includes my full check ride!

I also bought other books / resources, but these are effectively the only ones I used for my PPL, and I got 100% on the knowledge exam and also crushed the check ride. Best of luck in your journey!

Isn't WJ Wagtendonk's 'Principles of Helicopter Flight' still considered 'the bible'? ISBN-13: 978-1560276494

The previously mentioned Helicopter Flying Handbook is probably your best bet for free reading material.

If you don't mind spending a little money for reading material, I recommend:
(book title links to amazon)
[Principles of Helicopter Flight by W. J. Wagtendonk] (https://www.amazon.com/Principles-Helicopter-Flight-W-J-Wagtendonk/dp/1560276495) for around $25.
[Cyclic & Collective by Shawn Coyle] (https://www.amazon.com/Cyclic-Collective-Shawn-Coyle/dp/0557090660/ref=sr_1_1?s=books&ie=UTF8&qid=1497929356&sr=1-1&keywords=cyclic+and+collective) for around $45.
Another good one from Shawn Coyle is The Little Book of Autorotations which focuses only on autorotations.
I own all three, and they're all good, but I would recommend Cyclic and Collective. The book is packed with damn near everything you could possibly want to know about helicopter flying. It also helps that Coyle's writing style helps hold the reader's interest even through dry and technical subjects.

A good book on the basics of helicopters and how they work:
http://www.amazon.com/Principles-Helicopter-Flight-W-Wagtendonk/dp/1560276495

A good textbook directed at engineers studying helicopters:
http://www.amazon.com/Principles-Helicopter-Aerodynamics-Cambridge-Aerospace/dp/0521858607/ref=sr_1_1?ie=UTF8&qid=1372342685&sr=8-1&keywords=leishman