Why can't some images in the pdf be viewed on iBook of iPad or iPod Touch?
I'm going to attempt to answer this as best I can without detailed examples...

The screenshots and illustrations (which were most likely placed as EPS or Illustrator files) could be in some format or be corrupted in some way that the iBook PDF engine simply chooses to ignore the image (and is unarguably better than simply crashing).

PDFs are surprisingly complex documents that can be made in a number of different ways. Essentially, PDF documents are streamlined Postscript, which is a programming language unto itself. Support in PDF viewers for various features varies according to who the developed that particular viewer. 

We (I) don't know what engine Apple uses to read PDFs (i.e., open source, licensed, homemade), but I would be willing to bet that if those PDFs were opened in Adobe Acrobat, that there would be no problems viewing the art (but please don't ask me how to open iBook-sourced books in Acrobat because I'm not suggesting you do that and I have no idea if for no other reason that I don't use iBooks).

Update: @Jadasc points to a definite possibility to the problem. If the art was, indeed, placed in the layouts as JPEG2000 files, then that's either bad prepress practice or the books you are looking at are ePub files. In either case, without direct access to the books to fix them, I'm not sure what can be done about it.