Also, the description of the boolean satisfiability problem isn't the boolean satisfiability problem at all, but just what we might call the boolean evaluation problem, or a version of the circuit value problem, which is certainly in P.
I have no idea what's going on in the lambda calculus excerpt further down the page, in particular substituting (λx.x x) with (x x)? There seems to be a fairly big misunderstanding here. And lambda calculus isn't reduced in any particular order -- there are many ways to reduce the same term.
I was going to post the same thing about satisfiability. The excerpt about the Y combinator is also misleading. There is no practical way in which the Y combinator "finds the fixed point of any function"; (Y cos) certainly does not magically evaluate to 0.793085.
I think books like this are a good idea, and having self-taught people write them is also a good idea. BUT it looks like this particular book is in serious need of quality control.
(Also it should be split up into several parts. Any book that teaches both the Y combinator and how to configure zsh is... weird.)
I have no idea what's going on in the lambda calculus excerpt further down the page, in particular substituting (λx.x x) with (x x)? There seems to be a fairly big misunderstanding here. And lambda calculus isn't reduced in any particular order -- there are many ways to reduce the same term.