This depends on the level you're at. We'll need some extra info but in the mean time I'll give a list of books that are quite standard.
The standard text that is viable for advanced undergrads is Zwiebach but I'm not sure how far the current edition goes.
I used the book by Becker, Becker and Schwarz to get some general background for my thesis, I liked it more than Zwiebach's mainly because its more fast-paced. From the get-go you use group theoretic ideas and symmetry in a very fundamental way.
The next level would probably be Polchinski's books. They are hard, whenever I have to find something out (how does object X enter the theory) Polchinski's books are one of my last resorts because it takes a while to fully grasp what he writes (in my opinion).
Then there are the books by Green, Schwarz and Witten which have the same status as Polchinski's books, very thorough but hard.
A book that helped me grasp some ideas (nothing to exact) related to internal consistency of string theories is the book by
Theissen and Lüst. I've become partial to that one