[Fred Brackin]

There's some possibility of influence from Witches of Karres though the psi "Klaatha magic" is very high powered there. Also, Schmitz' Telzey Amberdon series.

Another possibility would be Niven's Known Space series. It at least has psis and has to have had some influence on Traveller.

There was some occaisional psi in the Van Rijn/Flandry universe of Poul Anderson and that's influence on Traveller would be difficult to deny.

Oh, forgot Alfred Bester's Demolished Man which may have had some influence on what Zhodani society would be like.