Moved to Forum: Original Link
Apple's iOS Human Interface Guidelines, a set of tips and rules for designers that was previously only available through the developer portal, is free on iBooks as of today.
Some more info here….
http://gizmodo.com/apple-just-put-its-app-design-bible-on-ib…
It has always been free. Previously available in PDF on the website, now available in iBookstore. Not a bargain per se.