I now compiled such a document listing all internal macros of LaTeX2e which are also useful for package authors. The work title is "List of internal LaTeX2e Macros useful to Package Authors" and I'm planning to release it on CTAN 'soon' (e.g. as '
macros2e') when its out of the beta stage.
The beta release can be found on my website.
There is also a feedback form and email address on the website.
Please do not hesitate to suggest more macros I might have overlooked, better explanations of already listed macro, report errors of any kind or anything else.
Update 26th July 2011
It is now on CTAN. The source repository is public. Please feel free to provide comments, patches or open an issue ticket if you find anything which can be improved.