This e-book is available free courtesy of Devontechnologies, from Take Control Books or on Amazon Kindle. Joe Kissell confirmed this.
So now there’s two books to consult, the downloadable pdf of DTP3 Help at https://s3.amazonaws.com/DTWebsiteSupport/download/devonthink/3.0.1/DEVONthink%20Manual.pdf
and Joe’s book at
https://www.takecontrolbooks.com/devonthink-3/
I see that the free Take Control book was announced in the forum about 10 days ago but I wanted to repeat the good news, along with the link to the entire DTP3 Help in pdf form.