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
Full title
Anth. van Leeuwenhoek, vereerend herdacht in eene korte levensschets en lofdicht
Anth. van Leeuwenhoek, worshipful commemoration in a short life sketch and panegyric