I learned about processes in unix by reading

S. Leffler, M. McKusick, M. Karels, J. Quarterman, The Design and Implementation of the 4.3BSD UNIX Operating System, Addison-Wesley Publishing Company, Reading, MA January 1989, ISBN 0-201-06196-1.

It gives clear and detailed descriptions of how the operating system works, including what processes are and how they are managed.

This book, 4.3BSD (and me) are quite old now, but the concepts are still clear and relevant.

There is a much more recent book by McKusick and Neville-Neil:

M. McKusick, G. Neville-Neil, The Design and Implementation of the FreeBSD Operating System, Addison-Wesley Publishing Company, Reading, MA. April 2005, (ISBN 0-201-70245-2).

The text of Chapter 4: Process Management, is available on-line.

Implementation details will be different for other flavours of *nix (e.g. linux) but the basic concepts of what a process is and how it is managed will remain consistent.

Update: corrected the reference to the old book.