Thanks to all for the advance work.
In case this is related to a more recent or the current app release of pdfexpert, i wanted to put a timestamp on the thread. writing to add to the confirmations that removing pdfexpert solved the problem of inability to download samples in ibook - just encountered on ipad2. Could download ibook samples fine up until now or at least up until around November 2011. The change was so abrupt I thought it at first a ploy to force ibook sales :), but it may be related to a more recent release of pdfexpert.
Action: removed readdle's PDFExpert app, closed and reopened ibooks and i was able again to download samples.
Different about my example is I have alot of doc handling (ezsign, docusign), office, print and pdf tools (readdle, pdfexpert, scan2pdf, save2pdf, print2pdf) installed. Removing first and only pdfexpert app by readdle solved this.
Googled this problem on a whisper of hope, found this thread and you all helped resolve.