Is there a printable version of a reference manual for all TeX primitives?

I think your best bet is Victor Eijkhout's TeX by topic.

Oh, and by the way, some of those “primitives” aren't primitives at all, but plain TeX macros.

Joseph assumes that "Buy the TeXbook" would not count as an answer, but I disagree.

I think that The TeXbook is very well-written and explains every primitive and plain TeX macro. It's well-worth the time to read it for any serious TeX user. The previous sentence is still true even if you never want to write a macro package or delve into the innards of TeX. It helps when macros become something you can understand rather than mystic incantations you utter from time to time.

The book TeX for the Impatient is now freely available, and covers Plain TeX quite well: http://www.gnu.org/software/teximpatient/

At one point in time, I recall seeing an announcement of the source of Plain.tex inter-woven w/ the appropriate comments from TeXbook.tex as a literate program, but that probably runs afoul of the copyright limitations of the latter (but raises the question of why plain.tex isn't plain.web)