[1]:http://pages.bangor.ac.uk/~mas010/nonab-a-t.html
[2]:http://pages.bangor.ac.uk/~mas010/


The notion of free crossed module was a major feature of JHC Whitehead's 1949 paper "Combinatorial homotopy II", in which he proved, using methods of transversality and knot theory, that the crossed module 
$$\pi_2(X \cup \{e^2_\lambda\},X,x) \to \pi_1(X,x) $$
is free on the characteristic maps of the $2$-cells. This theorem is sometimes mentioned but rarely proved in topology texts. Work of Philip Higgins and I showed how this theorem was a special case of a $2$-dim van Kampen type theorem, i.e. a colimit theorem, and the full story of this is given in Part I of the book partially titled [Nonabelian Algebraic Topology][1], EMS 2011. Thus the more general theorem determines
$$\pi_2(X \cup _f CA,A,x) \to \pi_1(X,x)$$
for $A$ connected, in terms of the  induced morphism $f_*: \pi_1(A,a) \to \pi_1(X,x)$, so that Whitehead's theorem is the case $A$ is a wedge of circles.