Kant retired from teaching in 1796, and spent his final decade working intensely on this manuscript. Despite his immense dedication for the project, he had occasional doubts in regards to its viability as a text--partially because his strength was waning, and he feared dying before he was able to edit the completed work to his satisfaction. Unfortunately, Kant's executor--clearly lacking understanding about a writer's process--took Kant's vacillation seriously enough to undermine the work, and unenthusiastically presented it to academic colleagues. The dismal result of this mismanagement was the disappearance of the text for fifty years. Guyer details the story of the manuscript's emergence and subsequent publication, including the nearly impossible task of organizing, interpreting, and translating the wealth of pages that comprise the modern version of the Opus. (I've quoted here from the Cambridge edition.)
History aside, the Opus Postumum is an astonishing work, with electrifying insight into physics, indicative of Kant's genius as a logician, but also momentous as an extension, and perhaps clarification and development, of his earlier metaphysical work. He continues his exploration of matter theory; constructs a definition of the concept of relation; discusses the science of nature, and the mechanical formation of the cosmos; discerns the subtleties of time, matter, and mass. Much too much to go into here, but it's well worth a look. After dozens of decades of obscurity, the whole collection of pages can now be purchased at Amazon for a mere $30.
I'll leave you with a fragment of text from an early leaf, what Kant may have considered a summation or notes in a margin, but what I'll call a poem:
Physics itself does not contain
a further transition from merely mechanical
to organic nature founded on the concept of purpose
which transition, and according to which causal laws these
purposes could be explained, exceeds the insights
of human reason
because physics itself here makes a leap, namely to
a nature which can be thought possible only through purposes
for no bridge is placed for us
to reach from one bank
to the other.