I've never looked for books on the AVR, but the data sheets are readily available from Atmel.
From page 27 of the 328 spec sheet PDF.
The device is shipped with internal RC oscillator at 8.0MHz and with the fuse CKDIV8 programmed, resulting in 1.0MHz system clock. The startup time is set to maximum and time-out period enabled. (CKSEL = "0010", SUT = "10", CKDIV8 = "0"). The default setting ensures that all users can make their desired clock source setting using any available programming interface.
The chip will run just fine at 1mHz straight out of the box, by unprogramming the CKDIV8 fuse you can get it to run at 8mHz.