are peer-to-peer networks still a good strategy to get free publicity around your book, or for testing a minimum viable product? Or are there other ways, more efficient...
When I give talks about content monetization, someone usually asks about piracy protection.
There is no way to protect against piracy.
My suggestion is always monetize piracy. How exactly you do this depends on your product.
P2P content provision can be used in many ways to boost marketing traction.
Big difference between free publicity + MVP testing.
And, I suggest you use Meetup. I tend to do all my MVP testing giving free Meetup talks to live audiences.
The questions you get will help you refine your pitch/copy + maybe your product/service/info design.
You might clarify your question a bit, based on where you are in your book's lifecycle + also your book title + topic.
Likely other people will have great suggestions based on your specifics.
I don't see why it wouldn't still be applicable. Peer-to-peer file sharing has been increasing ~50% ever 5 years since 2008 (source: https://goo.gl/6WN6hU).
There are also now additional tools that you can simultaneously take advantage of. For instance, although e-book reader sales peaked in 2011, there is a huge amount of traffic on the Amazon kindle store. Many authors use Amazon's option to provide a free electronic sample of their book, which I know from personal experience works. I often download free samples and then end up purchasing the book.