It is know that in order to have ${\cal N}=1$ 4D compactification of M-theory or 11D SUGRA, one has to take the internal space to be a manifold of $G_2$ holonomy. Moreover, such manifolds are considered as possible target spaces of Topological M-theory.
Could anybody recommend a review of geometry and topology of 7-folds of $G_2$ holonomy? I am a physicist, so the reviews written by physicist or at least for physicists are most welcome.