Lightbrown and Nada's "How Languages are Learned" makes a good introduction to second language acquisition and some of the linguistic theories that are out there regarding it. Without overloading you with details, the book gives you enough to make some more informed choices about the classroom.
They start from a basis set in first language learning and some of the ideas that have come from there, as well as the nature of how children learn their first language. From there, it is pretty much straight into second language acquisition, including 5 approaches to it, learner language, interaction approaches and styles, factors that affect second language acquistion, (such as learner beliefs, motivations and even a detailed discussion on the age of students), and a range of others. The book is wrapped up with the authors' responses to 12 common beliefs about language learning.
For those looking for more inspiration in the classroom, there are better books out there. This one will give you something to think about, and maybe use as a basis for adjusting one's technique, but it really deals more in the theory and how that impacts on our understanding of students. If theory, and a decent introduction to it, is what you are after, then might I recommend this one as a good starting point.
Unlike some, Lightbrown and Nada have avoided throwing in so much jargon that their book is rendered useless by most people. It is very accessible and easily read. There will probably be no headaches after this one. It is an interesting, well-written book.