I have become increasingly convinced that decent books from the nineteenth century will be more useful PSHTF than many recent ones. For starters, OHS and Rampant Lawyers were not worried about, and self-sufficiency was appreciated, so you're more likely to find information you will need on some chemical processes, how-to lists for many things - when it was assumed that you'd have to DIY from basic ingredients, rather than just press a couple of buttons over the internet.
Not to say that there aren't problems... Chemistry wasn't quite as advanced as today. In with the useful, you will also find the incredibly toxic - avoid any recipe that calls for White Lead, for example. You will find different measuring systems (particularly when they mix weight and volume measurements). You will find arcane names for things - Dragon's Blood refers to the sap of the tree Dracaena cinnabari.
A lot of modern books aren't as useful, especially when they assume you have ready access to parts you couldn't make by yourself.
I won't claim that books are replacements for all other preps... But if the difference between death by starvation, death by poisoning, and having plenty to eat is a small handbook on wild plants - what would you do?