The collection of J. Widekindi’s letters to M. G. De la Gardie, preserved in the National Archives of Sweden, has not received decent attention in the research concerning the origins of Widekindi’s main work, Historia Belli Sveco-Moscovitici Decennalis, which describes the events of the Russian Time of Troubles and the Ingrian War. Especially important in this respect is the last of the eleven letters, dated 15th March 1672 and dealing almost exclusively with Historia. From it we learn that the original text of this bilingual work was written in Latin (although the Swedish version was published first). Hence the status of the versions: the Swedish edition from 1671 is a translation, hastily made by several persons, while the Latin edition from 1672 is a revised and incomplete text of the original written by Widekindi himself. With the help of another letter we find out that Widekindi had started working on the book as early as by 1661. His main scope, clearly indicated in yet other letters, was to glorify his patron’s father, Jacob De la Gardie. But M. G. De la Gardie, who was aware of the project for a long time, gave his support to it only in 1666 (or later) — probably because it became quite relevant for his foreign policy. From the letters we also find out that Widekindi took care of Chancellor Axel Oxenstierna’s library from the middle of the 1650s. This fact, along with the extensive usage of one of Oxenstierna’s drafts in the first book of Historia, gives a clue to where Widekindi got acquainted with many of his sources. The catalogue of Oxenstierna’s book collection shows this suggestion to be correct.