Volumes 18, 19, and 20 of Alle de Brieven / Collected Letters became available at the end of 2024, completing the series begun in 1939. Download for free.
Which number?
Master list (pdf) of all 602 L-numbers, their Leeuwenhoek number, and their Collected Letters number
Robert Hooke's Micrographia published by the Royal Society in London. Leeuwenhoek's first letter to Oldenburg repeated and extended some of Hooke's observations.