I started to collect all the literature related to hash-based signature schemes here. The list is based on the list by Dan Bernstein from http://pqcrypto.org/hash.html. I re-read all the articles and added small summaries of the content that in my eyes is important for hash-based signatures. I also added several articles that I think belong to this list. It is actually interesting what you find if you go through all the old papers. Some problems and ideas I thought about were already solved / proposed in those papers. I also added all patents that I am aware of. Most of them are expired, only one patent covering some ideas that could be used to improve hash-based signature schemes in the random oracle model is not expired, yet. But this one has no connection to XMSS or Sphincs.
If you are interested in hash-based signatures — please have a look. I appreciate any comments or additions.