The LaTeX2e kernel macros are documented in source2e. An unfinished macros2e document lists all of its macros which are deemed directly useful for package authors.
If you need help with the underlying plainTeX macros and primitives (which lie even deeper than the LaTeX kernel) then I suggest the free books TeX for the impatient and TeX by Topic which are freely available on CTAN.
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I'd start with The not so short guide to LaTeX, and then have a look at
Which manuals are on your "TeX Reference" shelf?