Mr Letts was sales and marketing director for the FLB Group, which bought Letts Filofax in April 2013. He is the seventh generation of the Scots family to work in the firm, which was founded in 1796 in London, but has been manufacturing in Dalkeith since 1968, where it employs 260 people.
A post-mortem examination will be held to determine the cause of death, which occurred at Mr Letts's family home near Duns on Sunday, apparently following a cross-country run.
Mr Letts, who was an active outdoorsman and sportsman, leaves a wife and three children aged 11, 13 and 14.
In a statement to employees and clients, FLB Group chief executive Gordon Presly said: "Charles was an immensely popular and likeable individual with many close friends and acquaintances.
"He was a dedicated and hard working professional manager whose affable and gregarious demeanour appealed to everyone he met.
"He will be hugely missed by his family, his friends and all his work related associates."
FLB manufactures over 15 million diaries a year in Midlothian, and exports to over 75 countries around the world.