Here are some other currently or previously available books on Symbian OS programming:
Symbian OS C++ for Mobile Phones
Author : Richard Harrison
Publisher : John Wiley & Sons, Ltd.
ISBN : 0470856114
Programming for the Series 60 Platform and Symbian OS
Author : DIGIA Inc.
Publisher : John Wiley & Sons, Ltd.
ISBN : 0470849487
Symbian OS Communications Programming
Author : J. Jipping
Publisher : John Wiley & Sons, Ltd.
ISBN : 0470844302
Professional Symbian Programming: Mobile Solutions on the EPOC Platform
Authors : Martin Tasker, Jonathan Allin, Jonathan Dixon, Mark Shackman, Tim Richardson and John Forrest
Publisher : Wrox Press
ISBN : 186100303X
Programming Psion Computers
Author : Leigh Edwards
Publisher : EMCC
ISBN : 0953066304
Out of print, but now available online as a free download in PDF (Adobe Acrobat) format at http://www.emccsoft.com/devzone/ppc/.