What are good books to start and learn about Portal Development ?
I am looking for books and considerations to keep in mind which would help in designing and developing Portals like igoogle.com, www.yahoo.com etc and other similar public facing portals
It really depends on the portal engine but if you are interested in Java portlets (JSR 286) I would recommend JBoss PortalServer Development it's by far one of the best
If you are going to try to design a portal that is so large then you may want to be familiar with the concepts in "Service-Oriented Architecture", by Thomas Erl, as you will be writing a distributed system, where parts may be in different companies, but they need to work as one system.
As you begin to develop, and you start to get an idea about scale you will run into issues, and these will bring up needs for more information, such as using memcached, but that may be lower level than you want to go.
I think the difficulty is how low level do you want to go?
Do you need to go into detail about something like Hadoop, http://hadoop.apache.org/?