I've noticed that the authors of "NetBeans Platform for Beginners" have started exposing the number of sales they have achieved. Below, notice the '304' (which will probably change quite quickly) at the lower left end of this screenshot:
That's pretty good since the book has only existed for a few months and developers tend to share books they buy in PDF format. That probably means there are 300 teams of software developers around the world who are using the book, which is pretty awesome. (Though it would help the authors significantly, I'm sure, if individual developers on teams would buy the book, rather than sharing one between them. Come on, let's support these great authors so that they'll write more books like this.)
Also note that there is a set of reviewer comments on the page above:
Plus, the book is updated at the end of each month, so it continues to grow and improve from month to month, for free for everyone who has bought it.
If you've read the book and want to contribute a review like the above, contact walternyland @ yahoo dot com.
Great work, guys!
For anyone out there who hasn't got it yet: https://leanpub.com/nbp4beginners