Stephen Krashen provides his book Second Language Acquisition and Second Language Learning free of charge.
Language Contact and Bilingualism covers the a variety of topics, including the sociology of language, language shift, bilingual education, and SLA.
Where Immigrant Students Succeed is a free online textbook that discusses worldwide issues related to immigrants and international students.
Advanced Writing in English as a Foreign Language delves into topics such as SLA, writing pedagogy, feedback, and CALL.
Teaching Children to Be Literate explores models, approaches, and practices of teaching reading. While this is not a book written with ELL's specifically in mind, the book provides insight that would be relevant to any reading teacher.
Effective Online Teaching explores the implications of education on the internet, and how to most effectively teach online.
Practical Ideas for Teaching Writing as a Process, originally commissioned by the California Department of Education, provides an eight-step plan to teach writing as a process.