Most seem to agree that this is THE Pynchon book. Definitely not a quick,light read, but there IS a plot which picks up pace after a while. And what a plot! The most prominent theme centres on one Tyrone Slothrop, an American in England, who was raised in Germany in the decades before WWII, and was exposed in some sort of Pavlovian conditioning experiment (conducted by one Laszlo Jamf) which left him with a sensitivity to a compound which turns out to be present in the V2 rockets raining down on London. 'Pavlovian conditioning' refers (and this is very crude, I realise) to the pioneering work of Behaviourist Psychologist Pavlov(funnily enough) who studied the effect, probably long known to dog and horse trainers, whereby the subject is given a reward for some 'thing', then eventually the subject will perform the 'thing' in anticipation of the reward. It is noted by British boffins and secret service types that every time Slothrop has a sexual encounter a V2 lands not long afterwards, and he is held in a 'facility' sort of like a a Bletchley Park (where Turing et al worked to break the nazi Enigma code), dedicated to occult and psychological warfare, to determine whether he is actually anticipating the stimulus, and therefore predicting V2 strikes. For the first half, or even two thirds, of the book the focus shifts between different characters and locations who, at first, seem to have no connection but WWII, and whose relation to the main plot isn't made clear, but they all start coming together in the most entertaining way as the location shifts to newly, partly,liberated Europe, when Slothrop escapes and heads to Germany to find Jamf (I can't remember why, to be honest), and a 'team' is sent after him to castrate him. It actually becomes quite gripping, and for a finale, he brings all the characters together in a scene so hilarious and brilliant it's the only time I've ever felt like giving a book a round of applause. That scene is obviously his homage to James Joyce, being very reminiscent of the famous chapter in Ulysses where Joyce introduces a series of disparate characters going about their business, apparently unconnected, and then ties them all together by having a character take a coach trip through Dublin and encounter them all. Pynchon does it with a slapstick balloon chase.
The writing style is stunning - practically every page would shame the entire oeuvre of most modern poets. I have known more than a few compulsive talkers in my life, whose thoughts are always rushing ahead of them, whose every word suggests another word. Pynchon is like this, but elevated to genius; everything has ramifications, and the ramifications have ramification, some just tangential, some linked to themes which recur throughout the book, but it does, as some reviewers have noted, make it heavy going sometimes, especially at first. It's wrong, however, to see these flights of fancy as interruptions to the plot; they are what Pynchon DOES - brilliantly - his unique talent.
And what are the overarching themes of the book? Well, what is Gravity's Rainbow? Time? Einstein's theories as the occult faith of the twentieth century? I can't remember if the phrase 'Gravity's Rainbow' actually occurs in the text[afterthought: see the Amazon reviewer; I may have missed the obvious], but it's a suitably indeterminate title for a book which seems to me, like its predecessor 'V'(a harder read, I found, and so do most people), to be essentially about highlighting the fundamentally irrational and even occult basis for much of twentieth century behaviour, something we can see clearly when we look at say the fifteenth century, but less so when it gets closer to home. But don't expect the obvious.
Many writers have tried to advance on, or just emulate, the early modernist experimental writers like Joyce, Virginia Woolf, William Faulkner, to try and to do something more than narrative, and end up doing less; many have tried to write the Great American Novel, or the Great Successor to Ulysses; most seem contrived and pointless, without any real reason to be but Pynchon is a real original, inspired and authentic - also a bit awe-inspiring. A reading experience way beyond the routine; Gravity's Rainbow is so good it could persuade me to try 'V' and 'Mason and Dixon' again. THAT GOOD! THIS, for all its faults, is the Great [late] Twentieth Century Novel!