A common use case for PXM is to automate the creation of "pitch book" or RFPs - which basically pull together a sum of information (resources, articles, etc.) into a single publication on the fly.
Julia Gavrilova has written a great blog posting and guideline on everything you need to do in PXM to create a complex pitch book project, from planning to code (not unlike our own Jetstream APS Cookbooks) for pitch book creation.
It covers some complex PXM topics in one document;
Anyone new to an PXM project, or even someone experienced but venturing into a topic like "flow" (non-fixed page length) documents should go take a look.
Julia's blog entry can be found here at her Sitecore Stories blog (PDF link at the bottom).
Custom XML renderers - to handle interesting cases such as image placement and scaling for non-standard image ratios Custom XML renderers - to handle interesting cases such as image placement and scaling for non-standard image ratios