In 1928, J. W. Alexander published a paper “Topological Invariants of Knots and Links” in which he defined a polynomial invariant of knots and developed new insights including the braid relations. There are several ways to look at these invariants, some of these use the knot group previously defined by Max Dehn, but there are also various combinatorial methods derived from Alexander’s original one. One of the best known methods is via Fox derivatives and is described in the classical text by Richard Crowell and Ralph Fox.
R. H. Crowell and R. H. Fox, Introduction to Knot Theory, Springer, Graduate Texts 57, 1963.
Various approaches to the Alexander polynomial are described in introductory texts such as