The odds are good that your bike mechanic uses Park Tools. The people behind the tools wrote this repair book and it is an excellent guide to understanding the various bits on the bicycle, how they work and how to fix them when things go wrong.
The book contains straightforward instructions and many clear pictures. It mostly manages to present adequate information without being too overwhelming to the novice mechanic. By focusing on the tools needed to do a task, the book may convince the reader to buy more equipment (no surprise there), but it also is a useful guide for helping a rider decide what jobs are best done by their local bike shop. For example, while a flat tire is a task easily accomplished with simple tools by a home mechanic, replacing a broken drive-side spoke may require the spoke, a cassette tool, a chainwhip and a truing stand. All these things may be found at your local shop, along with mechanics skilled at wheel truing. While some handy folks will joyfully invest in the tools and time to make the repairs, others may just as wisely opt to pay a mechanic to do the job.
The Big Blue Book is currently in its second edition and is reasonably up to date. Reading a single book won’t turn someone into an expert bike mechanic, but for readers looking to learn more about the workings of their ride and the steps involved in the diagnosis and repair of problems, the Park Tools Big Blue Book of Bike Repair is an excellent resource.