The first book I bought by Harold Davis was on flower photography, and his techniques have transformed my approach to this, and many other, photographic subjects.
Then I bought The Way of the Digital Photographer. It's rare for me to buy books, because I normally do my learning from online sources. Harold's philosophy and teaching are clear and easy to follow. Even when he changes gear and becomes more technical, he explains his methods in a step by step fashion that's very successful. He's humorous too, which helps!
What I especially like is that he gives worked examples of every technique, and, like every good teacher, moves from the simple to the complex, and from the familiar to the unfamiliar, recapping at every stage. As an ex-teacher-trainer myself I'd say his writing/teaching is impressive.
Then along came Harold's Monochromatic HDR Photography. Which I bought! I thought I had already learned lots about post-processing from his previous books, but Harold takes it to a whole new level - with stunning results.
All the books are must-haves for those photographers who want to produce the very finest work that the digital negative contains.