The Foundation's website has the best long biography of Donne we've seen online.
This site includes a huge selection of Donne's poems, sermons, and essays, all of which are brilliant.
This painting by Giuseppe Maria Crespi, titled "The Flea," could almost serve as an illustration for the poem.
Did you ever notice that fleas look like alien creatures?
If you really want to be a Donne scholar, you should get your hands on this multivolume edition of his collected works. It's the authoritative version.
This book is apparently an example of "microbial history." It describes how the first outbreak of the Bubonic Plague (carried by fleas!) changed the history of Europe.