The Raspberry Pi is deceptively simple. Plug it in, boot it up, and use it as a personal computer, or attach a million gizmos and modules and invent something new and amazing. Either way, what it can actually do is not simple, and you should know exactly what the Raspberry Pi hardware is all about. Raspberry Pi Hardware Reference, from Mastering the Raspberry Pi, is the hardware guide you need on your desk or workbench.
Every detail is covered: from power to memory, from the CPU to working with USB. You'll find all the details about working with both wired and wireless Ethernet, SD cards, and the UART interface. The GPIO chapter is invaluable, covering power budgeting, access, and even small but important details like the correct usage of sudo when working with GPIO pins. You'll also find details about the 1-Wire driver, the I2C bus, and the SPI bus. If you need to know anything about your Raspberry Pi's hardware, you will find it here, in Raspberry Pi Hardware Reference.