XPath is a general language so there is a lot of docs about it. This is a good one:https://www.w3schools.com/xml/xpath_intro.asp
Mendix has actually quite some documentation on XPaths. Go to docs.mendix.com and search for XPath. On the right of the screen use the checkboxes to narrow down your selection to Mendix 8 documentation.
Start with https://gettingstarted.mendixcloud.com/link/module/22/lecture/122 and the six other lecture in the same path. Also useful:
Be aware of the different uses of Xpath, sometimes you get entire paths and XPath-functions at your disposal, like in the Retrieve-action, other times an XPath is only one association deep and no functions, like in a decision.