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To learn more about hacking the iOS operating system, the low-level nitty gritty of jailbreaking techniques, and other approaches to attacking the device’s operating environment, check out the iOS Hacker’s Handbook by Charlie Miller, Dion Blazakis, Dino DaiZovi, Stefan Esser, Vincenzo Iozzo, and Ralf-Phillip Weinmann (Wiley), 384 pages.
Although not directly related to iOS, you may also find the following books to be of great benefit: