I'm really glad this is being discussed :) Here's my $0.02...
For technical books, I think the right amount is about 10% to 20% of
the length of the planned book. Chances are, this is about 40-50
pages or about 2 chapters.
For computer programming books, in my opinion the test should be "will
this save an advanced reader 1-2 hours?" If your book saves an
advanced reader 1-2 hours, you'll save a newbie reader 10-20 hours.
Given how much we developers earn, if you save an advanced developer a
couple hours the book is already easily worth the money, even if you
didn't finish it! (And if you save the newbie 10 or 20 hours, that's
also worth it, even at minimum wage :)
Now, it's really hard to follow this advice. Years ago before Leanpub
existed, I self-published my first book in-progress. However, the
first version I released was almost 200 pages! Even though the book
ended up over 500 pages, the 200 pages amount was *way* too much. I
should have shipped much sooner...
As Aaron said, just be upfront on your book landing page about how
much is currently done and what you currently have planned.
-Peter
--
Peter Armstrong (@peterarmstrong)
Founder: Leanpub, Ruboss | Author: Lean Publishing, Flexible Rails, etc.