Stockfish is open because is a book on chess engine development, but instead of being presented as a collection of papers or as an "how to" documentation is presented in form of actual source code that, in my personal opinion, is the best way to present / teach software related stuff.
The source code book of Stockfish is the worst way to teach the "secrets of top chess engines". After reading through all the volumes of the Encyclopedia Britannica, what we get in the end is the end.
The Ippolit authors know how to write a top chess engine but they code it in an almost inscrutable style of C. It is nothing evil. They are just playing a game - "if you want to know the secrets you don't have it easy".
Why is Komodo a top engine ? Don rewrites a new chess program and it easily make it to the top because he knows the "secrets". It is the same with Naum and some others.
Bob Hyatt said that there is a gap between Crafty and Stockfish but "not an insurmountable gap". I say the gap is insurmountable. The gap exist because Crafty is NOT Stockfish and as long as Crafty is "not" Stockfish it may never close the gap. Why?
I'll ask Bob Hyatt : "Is the evaluation of Crafty significantly different from that of Stockfish ?" If he says it is a clone evaluator of Stockfish, then I quit - I don't know why the gap. But if he says "yes, the differences are quite many and substantial". I will then say "There you are, you have answered yourself where the gap is". A top engine's evaluation is not simple and all the many different aspect must fit as "...Everything should fit together perfectly". If Crafty is to want an elo 3000 evaluation and at the same time substantially different from that of Stockfish, then it must be "substantially different in a smarter manner".
It is very difficult to beat the evaluation of stockfish as these top engines have pushed chess programming to its limit starting from Rybka. BB mentioned in a post somewhere that (probably) what Vasik contributed to computer chess is scientific testing - but he must know fairly clearly what to test.
If Michael Sherwin were to ask "Are you sure you got it right - that evaluation is this important ?". My answer is in a question : "If you reverse the sign of the evaluation of Stockfish and play it against TSCP, which is the stronger program ?".
Don't believe when you're told "There's no free lunch!" There is Linux.