Write Portable Code: An Introduction to Developing Software for Multiple Platforms, by Brian Hook, is a short book with 18 short chapters describing the facets of computer hardware and software that can affect the portability of code. As the subtitle indicates, this is an “introduction” to writing portable code, so there are few specifics but lots of general information. This book should be useful if you have no experience in dealing with portable code, or if you are learning a low-level language like C or C++, where portability issues are more common than with high-level languages like C# or Ruby. Knowing about portability issues is important not only for writing portable code but for porting existing code to your platform, so I would recommend this book to anyone that doesn't already have experience in this area.