Willow collectors are fortunate to have many books as resources for learning and identification. Most of these books can be found online in general searches or on such sites as Amazon and eBay. They may also be available on used book sites. Some may be out of print and more difficult to find. Note, price guides are not up-to-date and may not reflect current values.
In addition, there are books whose focus may not be exclusively willow, but they do feature willow in some capacity and therefore may be of interest to and a great resource for collectors.