PNG The Definitive Guide and Compressing with pngcrush

I just read that O'Reilly's Compression and Filtering (PNG: The Definitive Guide) is available online. The section on compressing PNG files was mentioned in the weekly GNU/Linux Journal newsletter:
I tried this out and the PNG file I had saved for the thumbnail was compressed with these stats:

Best pngcrush method = 7 for pngbook-cover-thmb2.png (22.56% reduction)

PNG Practical Compression Tips

In the first six chapters, I discussed a number of PNG-supporting applications and noted their limitations wherever possible; use that as a guide when choosing your tools, assuming you have a choice. Even if your program generally compresses PNG images well, consider using an optimizer such as pngcrush on everything when you're done;[76] definitely do so if your program is not known for its compression performance. For converting GIFs to PNGs, the dedicated gif2png is the most capable solution, even given its permanently beta version number; it preserves both transparency and embedded text comments.

Compression and Filtering (PNG: The Definitive Guide)