Mobipocket will not read plunker version books. You'd need to get a Plunker viewer installed on your pocket pc.
There is no standard for eBooks and no standard ebook software that will read all the different ebook formats. There are quite a few different readers available that read electronic books. The readers are limited to specific file formats, such as:
Microsoft Reader reads .LIT formatted books
Adobe Reader reads .PDF formatted books
Mobipocket reads .PRC formatted books (Mobipocket's desktop software can import PDF, HTML, TEXT, CHM, and OCF formatted books)
Plunker is yet another format and you can download a
free pocket pc viewer here.
The readers can only read books that are formatted for them.
There are conversion tools available that will convert books from one format to another format, but some of these tools are controversial because they circumvent a book's DRM. Some software, like Mobipocket's desktop version, can convert non-DRM books from one format to the Mobipocket format. You'd have to search the web for these kinds of tools.
There are plenty of DRM-free books that have been converted to many commonly used formats from project gutenberg books. Here is a website that has converted many gutenberg books into Mobipocket (and other) format and offer the books for free:
ManyBooks.net
Feedbooks.com
Because i don't want to install a bunch of different ebook readers on my pocket pc, I've decided to look for books only in the formats that I can read on the viewers I have installed on my pocket pc (Mobipocket & Adobe Reader). Most of the time, if I look hard enough, I can find a version of a book in one of these formats. I have Microsoft Reader installed on my computer, but I do not use it on my pocket pc.