This site has a lot of books on strength and mass gaining in the pre-steroid era. Some of these are barely accessible and you can view them for free

Its cool to see how the old guys trained and its very interesting how similar things are compared to now. Check out Olympic Weight-Lifting & Body-Building for All by Jim Halliday towards the bottom. Its cool to see how the improvised to do cleans off blocks, % prescriptions, assistance movements, etc.