I'm not saying the book is useless, just overrated. I'm sure there are bits of information there that one couldn't find in another resource, but to put it up as you did as the first resource someone should consult isn't great advice. For establishing, promoting and maintaining webcomics, there are tons of free online sources for that info, and I'd be fairly surprised (though may be wrong) if HTMW covers that in a comprehensive sense anyway.
If you're talking about the actual production of the comics themselves, there are again a lot of resources online that aren't as impossible to find as you make them out to be. The first one that jumps to mind is the ongoing
Making Graphic Novels from Jason Brubaker who does ReMIND which is a good instructional. There is also the sea of tutorials on deviantArt which are fairly easy to search.
And let's be honest, the only real difference between webcomics and regular comics, in terms of production, is output resolution, and even that isn't absolute.
There are also some fantastic books that come from better sources. Wil Eisner's books, Comics and Sequential Art and Graphic Storytelling and Visual Narrative, are a great place to start. Then you have Scott McCloud's series. Understanding Comics, Reinventing Comics and Making Comics are invaluable. Not only are McCloud's books something that should be read by EVERYONE who makes comics, but he focuses heavily on digital comics and he draws his information from a wide variety of areas, including Scott Kurtz. These books are more than just a one-sided instructional on the mechanics of comics, but they help you gain broader perspective on all aspects on our medium. All of these are available on Amazon too...yes, they're more than $5 but they're worth more than $5.
Forgive me for being blunt, but I don't care how many copies of it have sold and neither should anyone else. McDonald's does pretty well too, but that doesn't make it nutritious. Quantity doesn't equal quality and conformity only ever makes you one among millions. I suspect a large part of the reason, as well, this book is so popular among aspiring webcomickers is that it has the word WEBCOMICS on it and it's written by people who are famous by the standards of our industry. I'm sure if U2 got together and wrote a book called How To Make Records that it would sell a lot of copies, but I doubt that same number of people would record an album, and even fewer would do it well.
I'm not saying and won't say "Do not buy this book!" because more knowledge is always a good thing and in all likelihood it will spark an idea in your own craft, but I will say buying it before anything else is folly.