What was the most shocking and disturbing discovery made by Charley in the third level by Jack Finney?

The most shocking and disturbing discovery Charley made was that the note had been sent by his friend Sam who had recently disappeared.

Charley found out that he had exchanged a large sum of present time dollars and bought eight hundred dollars worth of old-style currency which he wanted to invest in the business in 1894 at Galesburg.

He also admitted that Sam was his psychiatrist who could not go back to his 'old business' (psychiatry) in 1894.