Here you may find an overview of the plug-ins comprising TextGridLab, linked to the API documentation. In other words: Start here.
You should also have a look at the TextGrid Developer's Guide and at the Reports R3.2 and R3.5 linked from the Reports page at www.textgrid.de for introductory information.
Reference documentation for the services etc. is available from textgridlab.org/developerdocs.