Obviously, you can try Googling which will give
you lots of info about microwave/RF couplers. would
narrow down ur search if you specify what coupler
you want, i.e. directional coupler, loop coupler,
probe coupler etc.....
in fact, some textbook
i would recommend are:
1) Microwave Engineering
by David Pozar
2) Introduction to Electromagnetic
Fields by Clayton Paul et al
3) RF components
and circuits by Joe Carr...
hope these help....