I also loved the Sontag journal, it was very inspiring. (Made me want
to run out and see foreign films and read Thomas Mann.)
John Cheever's journal was just published in paperback, I enjoyed that
one as well. If you can find them, doubt they're still in print, Jean
Cocteau's are a treat.
If you liked the Burroughs, you might also like Allen Ginsberg's. And
of course, there is always Ralph Waldo Emerson. It is fascinating to
read and see him working through the ideas that he later crafts into
essays.
I was inspired by Gail Godwin's The Making of a Writer; and Sylvia
Plath's journals were (for good or bad) a huge influence on my in my
teens and twenties, I recall reading them with great nostalgia, the
life a tortured artist; she had an intense life of the mind.
Hope this helps.
Shirley