Welcome Mallu - it's great to have another member from Kerala in SCF, and particularly from Trivandrum. Rod222 will be most delighted, because one of his interests is Travancore stamps. I'll leave comments on Travancore to him.
For
Cochin, the best source is still the Gibbons catalogues: either the Commonwealth & British Empire 1840-1970, or the India catalogue. They will give you a great deal of information, and they should be sufficient until you reach quite an advanced stage with your collection. There are two books that deal with
Cochin that I can suggest:
Stamps of Cochin by Stanley P. Luiz, privately published, 1972, and
Cochin Postmarks and Cancellations by G.B. Pai, published by Robson Lowe, London, 1974
As you can see, both books are long out of print. Neither is easy to find these days, but they do appear for sale occasionally on
ebay. Some other places you could try are Vera Trinder Books (
http://www.vtrinder.co.uk/) and Leonard Hartmann (
http://pbbooks.com/).I also discussed the stamps of
Cochin from the
George V era here:
https://goscf.com/t/8478, but I don't have enough useful material to go back further. There have been some very detailed studies of these earlier
Cochin stamps published in the journal of the India Study Circle. You can find it at
http://www.indiastudycircle.org/index.html. It's well worthwhile looking into if you collect these stamps seriously.