As a chef, I found this to be one of the most useful books I own - for technique it is always the first one I reach for. It gives you a very thorough grounding in the fundamentals - e.g. stocks, sauces, bread, pasta, baking - which you can then build upon. It is an excellent alternative to cookery courses. Compared to £20K for a cookery course, this book starts to look like extremely good value!
It covers subjects such as making bread, pasta and sauces in more depth than any other book I have seen, and it tells you what to watch out for, as well as what has gone wrong when the recipe does not turn out as expected. The book contains hundreds of valuable recipes but they are (very sensibly) for each fundamental technique (bread recipes, stock recipes etc.) rather than recipes for 'main courses' like most books.
The book is certainly not only for professionals - it is of just as much value to home chefs as it demystifies so many subjects and gives you so many fundamental 'building block' recipes.
Despite being vegetarian, I have found that most of the book is still relevant and seems to provide well for vegetarian alternatives - I was pleased to find a recipe for 'brown vegetable stock' for making gravies etc!
One aside though - if you only ever buy one cookery book, make it Leith's 'Cookery Bible' - it contains a lot of the information in the 'techniques bible' but also nearly every recipe you will ever need.
Many thanks to Leith's!