If you re new to RETS and RESO, here are some tools and resources that I have found to be helpful.
RETSMD.COM -- You can login to your MLS using anyone of the RETS versions (up to 1.9). This is a useful way to get a LIVE data dictionary and schema of a specific MLS.
RESO.ORG -- The RESO standards body. Additional RETS and RESO WEB tools/utilities, links to RESO code repositories and other useful information.
DMQL and DMQL/2 -- RETS uses the Data Mining Query Language (DMQL) to structure RETS queries. There are few resources on DMQL, HOWEVER this one has been the most helpful. A web search for DMQL or DMQL/2 will give you others.
There are other resources on GitHub or other repositories for RETS and RESO code frameworks. This list is intended to help developers who are new to RESO protocols, like RETS and RESO WEB.