Where can I find a hardware manual (documentation or a book) for Arduino board?
As far as I know, all hardware details are dispersed (or scatter) in different tutorials.
Arduino Stack Exchange is a question and answer site for developers of open-source hardware and software that is compatible with Arduino. It only takes a minute to sign up.Sign up to join this community
The Arduino Uno is little more than an ATmega328P microcontroller. In most cases you will be interested by the documentation of the microcontroller itself. Anyway, here are both:
Alberto (aka PighiXXX) has published nicer versions of the Arduino Uno pinout.
For the hardware part, you can refer to the schematic. It shows all the components so you can find their datasheets for detailed study.
But to me, if you're just using the board but not designing your own board or writing libraries for the board, I don't think this is necessary. Stick to application level or IO-level and only really dig into the hardware when you're blocked when doing things like using all timers, low-power mode, interrupt with Serial...