"Inside the Foreign Legion" by John Parker is the most balanced book on the Foreign Legion I've read. While Douglas Porch's seminal work still stands as the most thorough and well-documented book on the French Foreign Legion yet produced, it is very difficult to read. It took me months to wade through it. On the other hand, the virtual picture-book published by Yves Debay is more like recruitment propaganda--which is certainly not the case here.
"Inside the Foreign Legion" strikes a great balance between the two extremes, and is quite readable. While only scratching the surface of many important historical events (such as the few paragraphs given to the invasion of Benin, which Douglas Porch gives a whole chapter), all of the key points are present. The events are put together well, and one gets a very good overview of the Legion's history. Also, this book was published 10 years after Porch's, so there is quite a bit of information on more recent events.
Some of the book is geared towards potential recruits, but not a whole lot. If you're looking to join the Legion, this book will provide a lot of good information, but you are better off reading "Life in the French Foreign Legion" by Evan McGorman.
If you're looking to learn about the Foreign Legion for the sake of it, this book is excellent. I would reccomend you read this book first. If you then want to learn more information (on pre-1988 events), look to Douglas Porch's "The French Foreign Legion."