The book "Symbian OS" offers a comprehensive analysis of the operating system of the same name, which was specifically developed for mobile devices and smartphones. It covers the development history of Symbian, which descends from Psion's EPOC, and highlights the technical aspects that made it an important platform for mobile applications. The book discusses the various user interfaces such as S60, UIQ, and MOAP(S) and describes the role of the Symbian Foundation, established in 2008 to further develop the operating system as open-source software. The content is based on articles from Wikipedia and other free sources, providing a broad perspective on the topic. The publication of the source code under the Eclipse Public License, which was planned for 2010, is also addressed. This technical book is aimed at readers interested in the technical and historical aspects of Symbian OS.