The game puts randomized skill books in important locations, since books dont decay, you can take your time finding these. The hardest book to find is the advanced shooting book. Guns! guns! guns! which takes 25 hours to research. I found my copy in the bottom of the Ravine (exit east of Carter's Dam in Mystery Lake), bring a rope and some coffee for the climb back! The book was inside a railroad transportation cart thing that fell to the bottom of the abyss, it's back in the driver's seat. Dont forget to bring charcoal so you can map this place for the cartographer achievo.