Surprisingly undervalued books
Undervaluing Wittgenstein isn't really consistent with what I've read in American philosophy at least. If you go by objective metrics (which would be the Moneyball approach), he consistently tops the citation counts, and beyond that, is considered central to many areas. Probably only Heidegger gives him a run for most broadly influential 20th-century philosopher (though it's hard to compare directly, because they've been influential on quite different groups).He's been particularly influential on analytic philosophy via Saul Kripke, among other interpreters. In popularity contests, he routinely gets voted #1 most influential philosopher in polls of academic philosophers as well, e.g. in a 1999 poll of mostly UK/US academic philosophers (http://commonsenseatheism.com/wp-content/uploads/2011/02/Lac...) and in a Brian Leiter straw poll (http://leiterreports.typepad.com/blog/2009/03/so-who-is-the-...). The former one concludes that Philosophical Investigations is "the one crossover masterpiece in twentieth-century philosophy, appealing across diverse specializations and philosophical orientations".An interesting question might be who is undervalued on those lists: is there someone halfway down, or not on the list at all, who should be near the top?