For a general introduction regarding mathematical population genetics, please see:
"Ewens_Mathematical_Population_Genetics.pdf"
(Mathematical Population Genetics 1: Theoretical Introduction, by Warren J. Ewens, Springer, 2nd Edition, 2004)

The main recommended reading regarding fitness valley crossing is "Weissman09.pdf". 

Regarding the impact of population structure on fitness valley crossing, please see "Bitbol14.pdf".