The word "with" rather than "from" in the title is important. Relatively few people can earn a living as professional writers. A survey by the Authors' Licensing and Collecting Society (ALCS) found that the average (median) income of full-time authors was around £11,000 -- hardly a living wage. But if you regard your writing as part of a portfolio activities, which may include public speaking for instance, the picture starts to look less bleak.
The book is very readable. All the way through it feels like you're being given information by someone who has learnt the hard way what works and what doesn't. The "voice" is a personal one: I didn't feel like I was being lectured at.
There are plenty of ideas here about how to decide how you want to earn your money as an author, and links to lots of resources and websites.
One of the best things about the book is that Penn presents options as opposed to saying "you must do X". I've read those sort of books, and in the field of social media especially I have found that when I have tried to use a service that doesn't feel right for me at the time, I don't stick at it. And I certainly don't have the luxury of being able to spend hours on Twitter, Facebook, Google + and all the rest. In fact, the single most important piece of advice is that the most important thing you can do is write.
Although contract issues are dealt with, I think that ultimately one should always have a legal expert look over the contract. The reason is that no matter how savvy you might be, you can always be tripped up. For example, I was reading on the Writer Beware blog that if you sign a publishing contract that contains clauses to provide for arbitration in the event of a dispute, you are actually signing away your right to take legal action (see Signing Away Your Rights: Arbitration Clauses in Book Contracts). Now, I don't know if the same applies to contracts in the UK, but the point is that before I read that I might have assumed that having an arbitration clause was a good thing. I use the Society of Authors' contract checking service myself, and have found it to be excellent. I certainly wouldn't consider going it alone.
But that really is my only niggle. This is definitely a book worth buying.