The Silent Invaders by Robert Silverberg

Disguised invaders from rival extraterrestrial races, the Darruui and Medlin, struggle for control of the earth, and space travelers find themselves mysteriously transported to an unknown world

https://rapidgator.net/file/e0c0b71c37de9fdc1dd5ecb91319f95b/

http://nitroflare.com/view/C2210EF5B074740/