Before traveling, I like to go to used book sales such as the Phoenix VNSA to get books about the places where I will travel. In the case of guide books, they are frequently updated yearly with only small changes. Basically, any book in the smart phone era will still serve its purpose years later. Additionally, many of the images and maps have not changed at all for decades. In certain places such as China, the map may have changed significantly in ten years, but other places that is not the case. Most travel books are glue-bound which means you can cut them up for the parts you need and assemble a Frankenstein-like guide tailored specifically for your journey. By the time I leave, I will have assembled a guidebook for my journey for only a few dollars.
