It's also available in ZACH-LIKE [0], a free compilation of a bunch of his earlier flash games. Windows-only, however.
Now that the kickstarter is finished it's available for free as PDF (link in parent post)
"In addition to PDFs of the book, the digital version contains all pre-SpaceChem Zachtronics games and a bunch of early builds and prototypes, including some of the unreleased games depicted in ZACH-LIKE."