A novel by Neal Stephenson. The title is derived from the Necronomicon, a fictional work described in detail by H.P. Lovecraft.
Cryptonomicon is an interesting book in that it involves two (apparently) totally separate plot lines, with two disjointed sets of characters that don't intersect until well more than half-way through the book (a very large one, by the way, weighing in at 913 hefty pages in hardcover).
In the days immediately preceeding World War II two young men, Bobby Shaftoe, a descendant of an odd little family born of rugged Tennessee mountain men and lovely Phillipine women, and Lawrence Waterhouse, a brilliant but misunderstood mathematician, find their worlds turned upside down by the coming storm clouds of war.
Meanwhile, at the close of the 20th century, the uber-computer geek Randy Waterhouse likewise finds his world turned upside down by the combination of the breakup of a long-term relationship and the beginning of a new business venture to build an offshore "data haven".
The two plots develop and grow, and then finally merge in a complex story involving lost WWII Japanese gold hoards, secret societies, and, the common thread running through it all, cryptography and the deceptions, lies and intrigues that create the need for cryptography. The pace is fast-moving, although the context switches necessary to read what are almost two separate full-length parallel novels can be mind-numbing at times.
The novel is really more of a Tom Clancy-esque techno-thriller than a science fiction novel, even though it was written by a noted science fiction author. There are few social or technological projections forward -- this could easily be taking place today.