In a rather meaningless gesture, Apple uploaded a set of 6 iOS reference manuals to iBooks yesterday. The titles range from Cocoa Fundamentals Guide to iOS Technology Overview.
If you'd like to read a manual on the device you're developing for (the tautology is amusing), open iBooks and search for "apple developer". That should help find all 6 titles.
I'm calling this a meaningless gesture because you could already get these docs on Apple's developer support site. You can read them online or download them as PDFs. Start here.
I'm also bugged that they're releasing these docs in iBooks while at the same time hiding the info you need to actually make an ebook for them.