Yes, those are useful refreences, and of course everyone interested in low-level Perl should be familiar "perldoc perlxs" and its family (perlapi, perlcall, etc)
What's missing is the Perl Monks value-add: active threaded discussions, and (most importantly to me) lots of Real Life Working Examples you can look at as a starting point.
Hacking XS is insanely fun.
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