Megan, I think that the most honest agnostics are not pure materialists: they recognise the spiritual aspect of our being; and they are more honest about the limitations of science and about their deepest objections to God being of a 'spiritual' nature -- their 'right' to an autonomous will, their own ideas of morality, etc. This sort of agnostic will only claim for science the ability to prove that something is possible, or demonstrate probability: Bertrand Russell (in Our Knowledge of the External World) admits that science can never disprove the claims of mystics, it can only make a plausible case in defense of its own turf (which is rather ironic, given the absolute factual authority that many Christians ascribe to scientific theories).
I would recommend additionally to the above, Lewis' autobiography, and 'The Funeral of a Great Myth' in Christian Reflections, which gets the poetic appeal of evolution. I haven't read much by others listed but agree that things by Bertrand Russell, esp his autobiography and a book I hope to read, The Worship of a Free Man are probably more honest or at least revealing as to the real reasons why otherwise intelligent people reject God -- and they are the reasons that Scripture tells us. I found an essay I wrote years ago on the 'Greek Exercises' in Russell's autobiography, where he recorded his thoughts as a teenager working his way towards rejecting God's existence (a turn of phrase which reminds me of the futile escapism of not being grateful) -- I pulled/summarised a few things, below. However, I think it highly unlikely that most of us will meet an unbeliever of this much reflection: those I have encountered blindly adhere to a party line and aren't familiar with the more cautious statements of their own influential scientists and philosophers (I personally want to throw my hands up and cry, 'Why don't you agnostics try *doubting* something?).
At an age when most people are characterized by a preoccupation with the absurdly transitory, Bertrand Russell was preoccupied with immortality: either God was sovereign, and man, though an astonishing creation, was merely a mechanism for bringing about His will — or man had evolved without a creator, in which case he was a bundle of matter and chemical reactions. Either way, he was a pitiable creature, groping in a half-lit space between black holes: immortality is not for the mechanistic and the molecular . . .
Russell concludes that this truth is not conducive to happiness, and should not be widely disseminated, because the greatest happiness of the greatest number of people is the chief end of man. He identified man's misery with slavery to his 'primitive conscience', rather than to the sin it accuses him of: the morality which teaches us not to covet our neighbor's wife was necessary to the development of the species (not to develop which, would be sin), but is a mere encumbrance to the alleviation of the dying race. -- In other words, he refuses to worship the Creator because he prefers to worship the creature.
(Incidentally Russell doesn't think very highly of Spinoza -- he thinks he used logic merely to justify his more mystical moods. *smiles*)
about Bertrand Russell. He is worth the time to read. Unlike the "new atheists". Those "brights", as they like to call themselves, are pretty dim. 
