The Priory of Sion, Rennes-le-Chateau, Freemasons, Rosicrucians, Cathars, Isis, Black Madonnas, John the Baptist, Mary Magdalene, Hermes Trismegistus ... and much more are examined in The Templar Revelation.
If it sounds like the book is unfocused, that is likely to be your impression while reading it, at least for quite a few pages. There is a thread linking all these topics: Picknett and Prince are trying to trace the predecessors and descendants of the Knights Templar, who were cruelly suppressed early in the 14th century. They also probe the nature of the secret knowledge said to have been possessed by the Templars and their various offspring.
The connections aren't always easy to follow, and for awhile at least you're likely to find yourself at sea as the authors switch from one subject to another in kaleidoscopic fashion. In fairness, the evidence does seem by its nature to be complex and often ambiguous. Prepare to bring patience when you open the book; eventually, a sort of mosaic picture does emerge.
Picknett and Prince have certainly gone the whole nine yards in researching the material, quoting from hundreds of written sources and describing their conversations with people who might shed some light on the subjects, and they describe their own travels to relevant sites in the south of France.
Organization is not their strong point, but otherwise they are good writers who don't share the weakness of many occult researchers for trafficking in the obscurity and mystification endemic in the material they study. Further to the authors' credit, they appear to weigh the value of the evidence, and are not averse to rendering the odd skepical judgment on some of it. The numerous references are impressive, although a doubter could argue that quoting from multiple crackpot writings doesn't count for anything.
Whatever you make of all this -- and I confess I'm far from sure what conclusions to draw -- The Templar Revelation suggets convincingly that there has been throughout Western history an "underground" of individuals and organizations dedicated to preserving secret and often heretical beliefs challenging orthodox Christianity. And even if, in the end, you give this study a Scottish verdict of "not proven," you will respect the authors' sincerity and find this historical tour of occultism stimulating.
As one who (often) judges a book by its cover, I must highly commend the designer of the Corgi Books paperback. The main image is an embossed Templar seal overlaid with a version of the ankh; the title in gold foil raised lettering; and the entire cover laminated for an ultra-smooth, almost silky feel. The sensuous surface is a brilliant invitation to the mysteries with which the text deals.