"Learning eZ publish 3 : Building content management solutions" is probably the oldest eZ book out there. It's over 4 years old ( Released May 2004 ).
You'd get more up to date info from the eZ books
http://ez.no/store/books/ez_publish_advanced_content_management (covers eZ 4.0) & http://ez.no/store/books/ez_publish_content_management_basics ( bit older )
I doesn't appear that they cover extension development or using eZ publish as a framework. The "Learning eZ publish 3" does have some of this information. It should be still be relevant, although the API's will have changed but only slightly.The concepts remains the same.
BTW Felix wrote a good introductory article http://ez.no/developer/articles/an_introduction_to_developing_ez_publish_extensions there is also a lot of information on the eZPedia site http://ezpedia.org/
Many thanks for your input, I think I'll go for e-book option and use http://ezpedia.org and mentioned article to figure out differences when needed :-)