one of the interesting things about the baal shem tov is the he himself never wrote any books.
however there is a book sold today called “baal shem tov al hatorah” which is a collection of letters written by him,(in hebrew) organized by the parshah.
the book is transcribed on the Sefaria website (and translated to english) so you can see it if you want.
with all the debates going on chasidut these days its helpful to understand the seed torah which started it all.
hope this helps.