Authenticity is mostly in the eye of the beholder. Authentic to where and to whom? Who decides?
Further, tikka masala is not authentic to India at all, but was a construct of the Indian restaurant business outside of India. Still good and tasty. There's some links, a shrimp variation and other discussion you may find useful here:
http://www.cheftalk.com/forums/recip...mp-masala.html
Books
Julie Sahni's books are well regarded and I like them. I own a few. I prefer Madhur Jaffrey's books over Julie Sahni but Jaffrey adapts more freely to western realities. I also like Jaffreys' use of the pressure cooker which simplifies lots of this type of cooking.