'The Nature of Things' (in Latin 'De Rerum Natura'), is a lengthy poem written in the first century BC. It was intended to introduce Epicureanism to a Roman audience. It was evidently quite popular as many things were written about it. Unfortunately it disappeared and was thought to be lost until an intact copy of it was found in 1417.
The poem is largely scientific in nature, much more so than I was expecting. It's divided into six books, the first four of which comprise the reading for June. Each book is somewhat focused on a larger theme. (The copy found in the Great Books itself is a prose translation. I read a poetic translation by William Ellery Leonard. I found it quite readable, though there may be better translations available.)
The science is very interesting, especially for its time. Lucretius declares that everything is made up of atoms. Everything is made up of infintesimal particles and the void between them. From this he describes how the body works and argues that the soul is mortal and dies with each man. The scientific insight is fairly amazing for the time and you can see how it guided scientists when they finally had microscopes capable of seeing the small stuff. The spiritual stuff is well reasoned but runs into trouble because it's impossible to prove. (This is still a problem.)
I'll take some time and go through each of the books and also talk about the book 'The Swerve' by Stephen Greenblatt, which describes the refinding of the poem.