Here, I am also commenting on the Supplements to this book, which the publisher periodically issues.
The encyclopedia is excellent for a broad discussion of the hardware and software in computing. It has to regularly come out with Supplements because of the ongoing improvments in the field. All the major contributors to computing are discussed, with a good synopsis for each, explaining his or her bailiwick. The treatment is about 50-50 between hardware and software. The articles tend to discuss the main ideas, and the encyclopedia and its supplements should not be seen as even an attempt at a summary of vendors' offerings.
If you are new to computing and want a general introduction, try consulting articles here. Professionals in computing will already have specialised texts in their subfields, and may not really need this book.
