Sphere by Michael Crichton
An apparently alien spacecraft has been found at the bottom of the ocean, and when a team of scientists arrive to investigate, they find that it houses a bizarre spherical object. This is just the start of Michael Crichton's tense sci-fi/horror novel, which entirely revolves around the attempts to unravel this mystery. Things start to get very scary, particularly with the appearance of a giant squid that attacks the scientists' submarine.
I remember reading previous Michael Crichton novels and finding them to be a bit long-winded and full of overly technical speak, and this seemed true for this novel at first, which seemed very slow to get going. However, the book became increasingly gripping as I read further into it, and I found it hard to put down. I found myself loving the characters, who all felt very well-rounded, and there was a good sense of camaraderie between them.
The story becomes more intense quite early in when the characters find that they are stranded under the ocean for an indefinite period of time, and the book starts to feel very claustrophic, with tensions gradually mounting between the characters. I found the writing style to be reminiscent of Arthur C. Clarke's Space Odyssey series, and with the underwater setting there were inevitable echoes of 20,000 Leagues Under the Sea (Michael Crichton, aware of this, put in a few references to Captain Nemo). The best part of this story is the subtlety of how it is written, so when the apparent alien inhabitant of the sphere shows up, it's not a case of some bizarre monster appearing, but it is manifested through the mysterious "Jerry" sending increasingly sinister messages to the crew, and forming a presence not untlike 2001's HAL.
The best thing about it was that it built up to a very tense and gripping climax, which I enjoyed immensely. A recommended book.