There's also a perfectly good list of what you can and cannot count on in the ISO C standard, no need for secondary literature.
While the values of the bytes are not specified, the ability to get at them is, and a conforming implementation needs to provide this ability.
That makes absolutely no sense. Just think about endianness for example. Type conversions in C are extremely tricky, and in many cases are not guaranteed to be portable across different compilers even on the same architecture. There is a good explanation of what you can and cannot count on in chapter 6 of Harbison and Steele's C A Reference Manual.