Volumes 1 through 20 of Alle de Brieven / Collected Letters, the complete series. Download for free.
Master list (pdf) of all 602 L-numbers, their Leeuwenhoek number, and their Collected Letters number
A chapter in the book by Dr. H.L. Houtzager
Reinier de Graaf 1641-1673: In sijn leven Nauwkeurig Ontleder en gelukkig Geneesheer tot Delft, pages 41-52