I was just doing some digging and realised that the Achensee gets mentioned in a couple of interesting books:
John Buchan's The Thirty-Nine Steps
(1915) - could this have inspired EBD to visit it since we know she loved Buchan?
Anonymous - A Young Girl's Diary
(intro by Sigmund Freud) (approx 1919)
Friedrich Schiller's Wallenstein's Camp
names Buchau, but it is a different place, I think.
A woman either called Louise de la Rame or Ouida wrote a book called The Nuernberg Stove
which was published in 1909 and mentions Jenbach. Another book, Black and White Spirits
by Ralph Adams Cram, also mentions Jenbach while reciting a legend from Innsbruck.
Anyone found any others?