iOS Human Interface Guidelines describes the guidelines and principles that help you design a superlative user interface and user experience for your iOS app.
iOS Human Interface Guidelines does not describe how to implement your designs in code. When you’re ready to code, start by reading iOS App Programming Guide.
Once upon a time there was a book in Human Interface Guidelines (1987). This is no longer a book, but a website (https://developer.apple.com/design/hu...) that among other categories (macOS, tvOS, watchOS, and other technologies like the HealthKit) includes guidelines for iOS. It's getting updated relatively often, so it's the "go-to" place to be up to date if you are a designer, a developer or human factors enthusiast. The guidelines cover all levels regarding interaction, from architecture to visual element representations. My personal take-aways are the practical tips: "Try to maintain a minimum tappable area of 44pt x 44pt for all controls" [so that are easy to tap in mobile].
This is obsoleted by the newer version of the iOS HIG, of course, but it's an interesting historical perspective on pre-iOS 7 UI (for an example of how outdated this version is, the stocks screenshot shows AAPL at 123, before the 7-way split). It's a little annoying, reading past the cheerleading and propaganda ("Mail is one of the most highly visible, well-used, and appreciated applications in Mac OS X"), but overall the guidelines make sense and are a good alternative for developers that are unable to take an introductory HCI class.