lex is the original lexical scanner developed by Lesk and Schmidt; Paxson's improved version is flex. Similarly, yacc is Johnson and Sethi's original parser; bison is the GNU-equivalent written by Corbett and Stallman. Both are designed to be upward-compatible with the original while
adding extensions and improvements.
Original documentation by the authors of the tools themselves. These papers are quite readable and serve as an excellent introduction for familiarizing yourself with the tools.