Mine is the Conran Cookbook - I must have/have had over 100 cookbooks, but this is the one that I've used most over the years, and has the most flour and sauce on it's pages.
Only maybe half the book is recipes, the rest covers things like equipment, basic techniques, identifying good ingredients, cuts of meat, etc.
In terms of recipes, it's not a book I look at for inspiration or adventure, but it's the book that I think taught me most about cookery. The recipes work, and don't seem to gloss over things - I suspect because the book dates from a time when most British cookbooks were pedagogic.
Recently, the one that has had the most use - Snowflakes & Schnapps - a book of Northern European cookery (Scandinavia, Germany, Baltic states and Bohemia).