Apple has published an iOS security guide, a 20-page PDF explaining how Apple keeps iOS secure. This is not an end-user document; this is designed for businesses and large organizations who need to ensure that their data is protected. You won’t find any tips about setting passwords, but you will learn that, for example, statements such as these:
Every iOS device has a dedicated AES 256 crypto engine built into the DMA path between the flash storage and main system memory, making file encryption highly efficient.
Translated into English, that means, basically, “Your stuff is secure.”
This guide also covers network security and device access, and contains a useful glossary at the end if you want to get more info about the terminology used in security for mobile devices.