Letters of Marsilio Ficino (1433-1499) have been available ever since he died. Ficino wrote with a lot of people all across the world and held copies of every single letter he wrote. A few years before he died he even wrote an introduction to his combined letters himself! Soon after his death the first publishing of his letters was a fact. Being such a high amount, the letters were made available in 12 parts!
Also in English many letters of Ficino are available and I discovered translations in Dutch which are actually handwritten and published by a group of philosophers from Amsterdam. This book is no longer available it seems. Anyway, the Dutch Rosicrucian society Lectorium Rosicrucianum has two books with letters of Ficino and a nice introduction, which are the two titles here.
Besides letters Ficino wrote essays and a massive amount of books. Some are available in other languages than latin, others are yet unpublished in any way. A nice thing about the letters is that they are arranged by subject so you can read Ficino’s short explanation on a wide variety of subjects, going from everyday life to highly spiritual subjects.
For some reason these little books are not available from regular booksellers, while other books by “Rozekruis Pers” are, so you will have to contact them. I don’t know other available translations in Dutch, and for English ones you can of course check Amazon.