See Perl 6 and Parrot Essentials in the Perl 6 / parrot repository: "... A long time ago in a galaxy far, far away, Allison Randal, Dan Sugalski, and Leo Tötsch wrote Perl 6 Essentials , which later became Perl 6 and Parrot Essentials. The universe has changed quite a bit since then. Despite that, the first chapter is still very interesting because it's a slice of history about what people were thinking and why things happened the way they did. Allison recently arranged for O'Reilly Media to transfer the rights to The Perl Foundation so it could potentially turn into a community work that is up-to-date and relevant. The source files are currently marked as copyright of The Perl Foundation, but I suspect that will morph into something community-oriented. Allison told me that it's really under Artistic License 2, and that should work for now. ..."
"Perl Best Practices" by Damian Conway. Even though this currently applies to Perl 5, most of the principles also apply to Perl 6. This book is a semi-officially recommended guideline for people developing core Perl 6 modules and tests.