The Implementation Guides are actually quite thorough in their explanation of the workflow of EDI messages as well as the format.
Washington Publishing Company publishes all of these documents:
If your company is doing this work, it should already have purchased the necessary specifications.
In my own research I have found that most tools out there are either very expensive or does not have many features or relies too heavily on a database integration.
If there is a particular area of the 837P that is unclear to you, I would be happy to blog about it and point to good resources for further explanation. Anything that will encourage other users to try out the parser will be worth the effort.