Having just read the review by the esteemed Lisa Jardine (her "Worldly Goods: A New History of the Renaissance" is a great book)for the Washington Post, I felt compelled to respond.
She's rather harsh with it I think. I don't think Budiansky's history is any less accurate just because he is writing to an audience wider than us nerdy historians. It's just a lot less technical. I'm sure she misses all the footnotes and endnotes that we tend to live for, but I think that his narrative style gives his subject life and makes it a lot more fun to read than most history that is written for historians.
I think this book is a great introduction into the world of Elizabethan politics and espionage. Budiansky's work should not be compared to one such as Conyers Read's (still a great historian if a bit outdated), for they have completely different audiences for which they are writing. That said, perhaps Jardine was trying to say as much when she cited "Shakespeare in Love", but I think that's an unfair comparison... Budiansky takes FAR fewer liberties with fact.
So, if you are a non-historian, a casual reader, or if you're looking for a book for your undergraduate students, I recommend this book. If you are a history junky, and/or trying to find an authoritative work on Walsingham for graduate studies, you'll probably only want to use this book as a quick summary (it's an easy/fast read) of the carreer of Mr. Secretary Walsingham, if at all. There are certainly more authoritative books out there, but few as entertaining to read. And shouldn't history be fun to read?