Introductions to steganography from an information-theoretic standpoint
Here is a brief overview and introduction to steganography:

Christian Cachin. Digital steganography. In Henk C.A. van Tilborg, editor, Encyclopedia of Cryptography and Security. Springer, 2005

Since you asked for an introduction specifically for information-theoretic approaches, you might try this paper as a starting point for more detail:

Christian Cachin. An information-theoretic model for steganography. Information and Computation, 192(1):41-56, July 2004.

If you want a coding-theoretic perspective, you might also enjoy the following papers:

Yin Wang, Pierre Moulin.  Perfectly Secure Steganography: Capacity, Error Exponents, and Code Constructions.  IEEE Transactions on Information Theory, 2008.

Pierre Moulin, Joseph A. O'Sullivan. Information-Theoretic Analysis of
Information Hiding.  IEEE Transactions on Information Theory, 2003.

I do encourage you to also take a look at computational steganography (e.g., the Hopper et al. paper you cite in your question), as many of the powerful results are in the computational security regime