Damn cnblogs, no auto saving set by default, even worse than ****, can't believe it, lost half an hour worth of writing...
put it simple then,
That evening I had to help my wife with a book found on gbooks and we spent 15 bucks for purchase on gplay of the book and still hours of figuring out how to store it on local drive (however distributing it onto ipad and devices alike is simple and requires no smart brain); from what I did now it's clear that,
1. gbooks is different from gplay, former is for preview, and latter for purchase and complete reading, as far as I know;
2. That famous google book downloader opensourced on codeplex and the smart designed firefox + grease plugin + gbook downloader script don't work for gplay (at least they are not supposted to be for it);
3. One needs to have adobe digital edition (ADE) and calibre installed and .acsm file downloaded for the book purchase from gplay;
4. Calibre plugin won't help open the ADE drm encrypted .pdf at all;
5. A deDRM standalone application for epub (doesn't sound relevant from its name though) is crucial and makes all the magic happen. It stripes off the DRM from the .pdf converted from .acsm and saved in library folder by ADE and the output .pdf can further be processed in calibre (well, at this moment I'm not even sure if calibre is essential but it's no bad to have it) and saved as a normal plain pdf.