David Brin writes science fiction the way it should be written--with imagination, heroic characters, and the triumph of all that is good in the human spirit. "Sundiver" is a prime example of how good Brin's books can be.
Many cite "Startide Rising" as Brin's masterpiece. While "Startide" is a great book (Earth is actually my favorite book written by Brin), I do not think that Sundiver is a "weaker" effort. The two books are apples and oranges. One, "Sundiver," is essentially a mystery. The other, "Startide Rising," is more of an action-adventure book. I recommend them both. "Sundiver" really sets the context for "Startide" nicely.
"Sundiver" is a great mystery that kept me guessing until the end. It is filled with the kind of invention and personal treachery/heroics that make the Uplift series great. Brin has made a visionary world in his two great series.
I recommend this book.