As some of you know, the title of the Tractatus was suggested by G.E. Moore as a reference to Spinoza's Tractatus, a work that proved the impossibility of miracles. Wittgenstein's Tractatus reads like one of the hyper-geometric set of axioms a-la the Ethics, with lots of numbers and sections, giving it the sense of a long proof, though whether or not it was meant to be one is at least questionable.
Unlike the Tractatus, the Investigations is a set of sentences, paragraphs, numbered, perhaps reminiscent of Nietzsche's aphorisms, though much more explicated and logical. As Wittgenstein opens the Preface:
"The thoughts which I publish in what follows are the precipitate of philosophical investigations which have occupied me for the last sixteen years. They concern many subjects: the concepts of meaning, of understanding, of a proposition, of logic, the foundations of mathematics, states of consciousness, and other things."
He laments that he was unable to put his thoughts together into a into a single structure, saying "my thoughts were soon crippled if I tried to force them on in any single direction against their natural inclination." This smacks of the artistic, doesn't it? He even mentions "sketches of landscapes." If your mind is prepped by years of thinking you let it go where it goes, taking down the results.
A similar thing can be found in doing math. I've come up with math proofs in the same way. I think and think and then, sometimes late at night, a relationship comes to me, alas, nothing Earth shattering, but something new to me.
He felt he should put the Tractatus together with his new thinking, by way of contrast.
He ends characteristically:
"I should have liked to produce a good book. This has not come about, but the time is past in which I could improve it."
This last brings to mind two things: first, the long bouts of low fever Wittgenstein suffered with in a later age, but also a reminiscence of Russell; Russell says Wittgenstein burst into his room, pacing like a caged tiger, Russell asks, "Wittgenstein, are you thinking about logic or your sins?" Wittgenstein yells "Both!"