A press release should be completely devoid of your personal opinion on the book. Just the facts. Start by introducing the main character and explain how his life spins out of control when he becomes embroiled in this search for the Chalice. Don't give anything away. Maybe write two paragraphs that explain this mystery and the three or four main characters (the main guy, the main girl, the curator of the museum - maybe, and the albino dude hunting them down). Then write a paragraph about how Brown weaves this mystery together with a history lesson about Christ, Grail Lore, etc. Then write a paragraph or two bio on Brown. Include a couple of the capsule reviews that are in the book in another paragraph. Close it out by noting his other books including Angels & Demons (note how this is the first in the series of books featuring the main character from Da Vinci - I can't think of his name for the life of me - Robert Something or Other) and his other two books. And, if you can find it, mention what the author is currently working on. Try Googling for his website and see if he mentions what his next book will be. If you want samples of press releases about books, try a publishers' website like Bantam or Penguin. They should have a press section to their site that would include releases that you can use as samples.
If you'd like, you can e-mail me what you write. I work as a proofreader as part of my job responsibilities where I work so I'll check it out for you for grammar and spelling and whatnot.