There's a couple I like.
First of all, of course, the famous tome by Mitchell Heisman, guy wrote 2000 pages and then shot himself on harvard campus.
Mitchell Heisman, the man who shot himself September 18, 2010 in Harvard Yard, killed himself as ”Experimental Elimination of Self-Preser...
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On a more serious note, one that always resonated a lot with me is "Punched Out" by Will H. Moore
Assuming I did not botch the task, by the time this posts I will have been dead via suicide for several hours. Nope, that’s not a setup to a joke.[1] Why would someone who is healthy, employ…
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While not directly a suicide note but closely related, My Twisted World by Elliott Rodger is surprisingly readable and funny at times.
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Lastly of course, there's the philosophers. While not quite suicide notes, there's a pattern of them writing their magnum opus at a fairly young age and immediately commiting suicide once the work is finished. The following come to mind:
Otto Weininger, Sex and Character
Otto Weininger's controversial book Sex and Character, first published in Vienna in 1903, is a prime example of the conflicting discourse...
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Philipp Mainländer, The Philosophy of Redemption
In his central work Die Philosophie der Erlösung (The P…
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Carlo Michelstaedter, Persuasion and Rhethoric
Carlo Michelstaedter (1887-1910) committed suicide at the age of 23, days after completing this devastating treatise on the human conditi...
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All of these three are very well written and highly recommended, although they can be quite technical at times.
Of course keep the critical distance, especially with Sex and Character.
Wittgenstein said about it that if one were to put a negation sign before the entire work, it would reveal great truths.