A first wise best-friend to gently hold your hand for the first several months, if you are completely new and alone when it comes to both Symbian OS and the basics of the Symbian approach to design and development: from coding standards, memory management, platform security, string classes, GUI programming, client server architecture and asynchronous programming, all the way through to build environment practicalities.
Plus the forward on page xi has the most exciting, inspiring, thought provoking and insightful overview I've read on smartphones, and how they fit into the way modern life is evolving.