It's a Common Lisp book, not an Emacs Lisp book, but I think the concepts more or less apply. Basically (a . b) is called a cons cell -- a is the "car" and b is the "cdr". The cdr tends to be a pointer to another cons cell, making a list. That is, if b is a pointer to (c . d) and d is a pointer to (e . nil), you get (a . (c . (e . nil))), which can be displayed in list form as (a c e). The simple graphics in the link above really help for visualizing what's going on.