The contradictions, the little ellipses, the dashes, and the quick running thoughts all seem similar to me in the bit of Notes I've read and then The Subterraneans. Also Leo, like the Underground Man, is a bit lovably neurotic at times; both are self-conscious books which makes the narrative voice really interesting and strong. Underground Man keeps mentioning the reader collectively and anticipating the reactions of the reader, the "gentlemen." Leo agonizes over what he's revealing; its a confessional, he confronts painful memories of loosing love as objectively as he can by writing the book which weirdly is similar to the pleasure Underground Man gets from refusing to have his liver checked out for instance. Leo and Underground Man also suffer lack of money, tendencies to self-destruction, and their own neurotic-ness. Leo drinks and Underground Man thinks man is drawn to destructive things and is himself pretty disillusioned.
This is dull but its what my circa '81 Kerouac copy has to say about Dostoevsky: "Kerouac's most immediate source, however, as evidenced by the title, was D.'s Notes from the Underground, which propounded the idea that in our overly intellectual era, man can no longer face the reality without first shielding himself with numerous impenetrable layers of fantasy and dream. The role of the novelist, then, as D. saw it, was to unpeel this modern madness layer by layer though a continual forced confrontation with the most feared and forbidden - or as we say nowadays "repressed" - aspects of human personality. In this approach, D. was a precursor of Freud."
Leo looks for reality in dreams but those dreams ironically destroy the reality of his relationship with Mardou and Underground Man definitely wonders about free will and says that if man reaches the epitome of reason (blending reality with an imaginative, symbolic outcome), people will rebel against 2+2 until it's 5 and loveless logic is subverted so people can have feelings and imaginations and fantasies again... anyway Underground Man wonders a lot about free will until you wonder how free he is because he's finally rebelling against himself, causing himself pain for no reason except because he seems to prefer pain to numbness.