Heres a couple of books I have acquired in the last 2 months. Keep in mind that the books you choose should have far reaching purpose, by that I mean they will provide you with information in the Post Apocalyptic environment, not just in preparation for it.
Build Your Own Underground Root Cellar
Author: Phyllis Hobson
This is a small 32 page book that gives some pretty good detailed instructions on a root cellar.
Handy Farm Devices: And how to make them
Author: Rolf Cobleigh
A handy book but the instructions are small and not to detailed. It's more of an idea book from way back in the day. When there are no more hardware stores, this book will be handy.
FM 21-76 US Army Survival Manual
Ok, so I've had this for awhile, since 79 to be exact when they issued it to me. The stuff in there will NEVER get old. You can get this at Barnes & Nobles now, saw it there on display in front of the store and thought that was kind of ominous. There are plenty of knock off's with names that come close, so be wary.
Water Storage: Tanks, Cisterns, Aquifers, and Ponds for Domestic Supply, Fire and Emergency Use--Includes How to Make Ferrocement Water Tanks
Author: Art Ludwig
This is an awesome book. I was looking for ways to store large amounts of rainwater for my garden and this did it. The projects range from small to crazy large, but you need to learn to store large amounts of water if you are to survive. 1 gallon per day per person is the recommended amount just to stay alive, cook, and be healthy. What about your food source? The garden needs water too.
I try to buy my books used on Amazon, or at local used book stores. You can also try the couple of on line book trading sites as another source.