OFX is an open API for writing visual effects plug-ins for a wide variety of applications, such as video editing systems and compositing systems. It seems to have two interchangable names, "OpenFX" and "OFX". I prefer OFX for some reason. This book is actually very broad in scope, as it has to not only define the API, it will need to go into some depth describing concepts behind imaging applications that use it.
Do you write visual effects or image processing software? Do you have an application which deals with moving images and hosts plug-ins or would like to host plug-ins? Then OFX is for you.
This book assumes you can program in the "C" language and are familiar with the concepts involved in writing visual effects software. You need to understand concepts like pixels, clips, pixel aspect ratios and more. If you don't, I suggest you read further or attempt to soldier on bravely and see how far you go before you get lost.