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
great great-uncle Cornelis Cornelis van den Berch, Sebastian's half-brother, married Immetje Jans Hogenhouk, Sebastian's wife's sister