user18431: 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.

user13176: For the hardware part, you can refer to the schematic. It shows all the components so you can find their datasheets for detailed study.
UNO schematic
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...

user7508: 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:

The Arduino Uno product page contains the most basic specs, as well as links to

the board schematic
the pin mapping, but see below for a nicer alternative


the ATmega328P product page links to

the ATmega328P family datasheet, a 650 page authoritative document
the ATmega328P Automotive - Complete Datasheet, specific to the Uno's chip.
the AVR instruction set manual, for those who want to play with assembly
many application notes discussing specific uses of some peripherals



Alberto (aka PighiXXX) has published nicer versions of the Arduino Uno
pinout:


18431: <p>Where can I find a hardware manual (documentation or a book) for Arduino board? </p>

<p>As far as I know, all hardware details are dispersed (or scatter) in different tutorials.</p>
 13176: <p>For the hardware part, you can refer to the schematic. It shows all the components so you can find their datasheets for detailed study.</p>

<p><a href="http://www.arduino.cc/en/uploads/Main/Arduino_Uno_Rev3-schematic.pdf" rel="nofollow">UNO schematic</a></p>

<p>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...</p>
 7508: <p>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:</p>
<ul>
<li>The <a href="http://www.arduino.cc/en/Main/ArduinoBoardUno" rel="nofollow noreferrer">Arduino Uno product page</a> contains the most basic specs, as well as links to
<ul>
<li>the <a href="http://www.arduino.cc/en/uploads/Main/Arduino_Uno_Rev3-schematic.pdf" rel="nofollow noreferrer">board schematic</a></li>
<li>the <a href="http://www.arduino.cc/en/Hacking/PinMapping168" rel="nofollow noreferrer">pin mapping</a>, but see below for a nicer alternative</li>
</ul>
</li>
<li>the <a href="https://www.microchip.com/en-us/product/ATmega328P" rel="nofollow noreferrer">ATmega328P product page</a> links to
<ul>
<li>the <a href="https://ww1.microchip.com/downloads/en/DeviceDoc/ATmega48A-PA-88A-PA-168A-PA-328-P-DS-DS40002061B.pdf" rel="nofollow noreferrer">ATmega328P family datasheet</a>, a 650 page authoritative document</li>
<li>the <a href="https://ww1.microchip.com/downloads/en/DeviceDoc/Atmel-7810-Automotive-Microcontrollers-ATmega328P_Datasheet.pdf" rel="nofollow noreferrer">ATmega328P Automotive - Complete Datasheet</a>, specific to the Uno's chip.</li>
<li>the <a href="https://ww1.microchip.com/downloads/en/DeviceDoc/AVR-InstructionSet-Manual-DS40002198.pdf" rel="nofollow noreferrer">AVR instruction set manual</a>, for those who want to play with assembly</li>
<li>many application notes discussing specific uses of some peripherals</li>
</ul>
</li>
</ul>
<p>Alberto (aka PighiXXX) has published nicer versions of the <a href="https://upload.wikimedia.org/wikipedia/commons/c/c9/Pinout_of_ARDUINO_Board_and_ATMega328PU.svg" rel="nofollow noreferrer">Arduino Uno
pinout</a>:</p>
<p><a href="https://i.stack.imgur.com/Um1kq.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Um1kq.jpg" alt="enter image description here" /></a></p>
