Some of your questions are already answered within this
thread, especially the link by Adeel Ansari about servlet-portlet diffs provides interesting information. Do you have a specific question about servlets and portlets, that is not covered by this resource?
Only a few books about JSR 168 exist, one is "Building Portals with the Java Portlet API"
(Homepage) by J. Linwood, D. Minter (APress 2004). This one is intersting from my point of view. More books exist on specific portal servers. For IBM WebSphere Portal server I would recommend to get the relevant Redbooks, which are available for free (PDF, HTML).
Greetings Stefan