The classic paper in the US literature on tumor heterogeneity remains Gerlinger et al, in NEJM 2012 (here.) On May 11-13, there will be a (free) international conference in Washington DC on the topic. The conference is sponsored by the Menarini Foundation, the University of Hamburg, and MD Anderson.
See the website here and here the PDF brochure here. It's at the "Congress Center of the Old Post Office Building," which I believe is now Trump International Hotel.
New papers appear regulary across a range of cancers, e.g. new work by Morissy on medulloblastoma heterogeneity in Nature Genetics, April 2017, here.