Silent Warfare is probably the best introductory text available covering the subject of intelligence. It reads like a text book, but that's because it basically IS a textbook. It's a serious academic text rather than a cloak and dagger story. This is one to read for understanding rather than necessarily for pleasure.
The book is fairly short but covers all the bases in terms of types of intelligence, types of intelligence organisation, the various debates surrounding the subject etc. It is, perhaps inevitably, somewhat America-centric. British intelligence and the KGB stick their heads into the picture from time to time, largely to provide illustrative comparisons rather than as studies in themselves.
When making a point, the authors generally try to provide historical examples and comparison, which is helpful, especially for the beginner. It also helps to enliven the text a bit.
The book is extremely well sourced and many of the end notes contain further explanations and are extremely interesting in themselves.
The only thing I feel the book lacks, and this is a fairly minor quibble, is a bibliography. This would have been very useful, especially in what is intended to be an introductory textbook. A bibliographical essay with suggestions for recommended further reading would have been even better.
Quibbles aside, this is a very good primer and to the best of my knowledge there are no books on the market that can compete with it in terms of providing a solid academic introduction to the subject. People with a serious interest in intelligence would be well advised to follow this book up by taking a look at the works of Michael Herman, which provide more in-depth coverage (especially "Intelligence Power in Peace and War") and a non-American (in this case British) angle - though they may be a little heavy for the absolute novice.
To sum up, if you have never read an academic book on intelligence before this is the one to go for.