Countless English as a second language books have been published in recent years to meet the needs of a variety of audiences.

Teachers and educators represent the primary audience for books on the subject of second language education, but this demographic is by no means homogeneous. English as a second language books designed for K-12 teachers typically contain information that is pedagogical in nature. For example, complete units on a particular holiday or season might be provided with corresponding activities and daily lesson plans.
At the university level, much of the ESL instruction is based around writing instruction, particularly of the type that is required in early composition courses. These texts may also contain activities and techniques for use in the classroom, but texts for this population also are likely to contain theoretical information that situates contemporary ESL pedagogy in relation to earlier philosophies concerned with the teaching of English.
Finally, some ESL books that published have the second language students themselves in mind as their intended audience. Whether the student is a younger student who needs extra work at home or an adult ESL student trying to understand the specific challenges that speakers of her native language encounter when learning English, it can be safely assumed that there exists a book that can help.
The following list of texts is complete with brief descriptions and is organized by the books' intended audiences.
Obviously, the above selections are a small sampling of the immense universe of English as a second language books. If you discover a particular text you find useful, search out more works by that author or by any authors that are referenced frequently in the text. Bear in mind that there are multiple competing theories in the world of ESL pedagogy; you might need to do considerable research in order to zero in on the philosophies that work for you and your students.