Plack handbook has 24 useful short articles explaining the concept of PSGI and tutorials how to adapt Plack to the existing web applications. Written by the author of PSGI specification and Plack himself, this book has been considered a canonical reference for many beginners trying to learn Plack and also web framework authors trying to adapt PSGI.
Plack Handbook English edition is now available on Kindle, just in case you don't want to (or can't) use gumroad for some reason. Also available on Amazon.co.jp and all other countries.
Note that you will only get English version from Kindle, and if you want to get both English/Japanese in EPUB and MOBI, you can still buy from gumroad for the same price, $5.
Japanese edition has been submitted to Amazon KDP at the same time and it is still in review. The book will be available on Amazon.com, amazon.co.jp and all Kindle participating countries.