If you want to learn about SLA, the best starting points are Vivian Cook’s Second Language Learning and Language Teaching and Lightbown and Spada’s How Languages are Learned. Both are very readable and comprehensive, but not very detailed, so if you find something you want to delve deeper into, a good next step is Mitchell and Myles’ Second Language Learning Theories or Gass and Selinker’s Second Language Acquisition: An Introductory Course. These are readable only if you already know a little about the material they’re writing about; don’t read them cover-to-cover: even the authors suggest you jump around the book. Then, if you really want to find out more about an approach, a theory, a teaching model or whatever, dive into Ellis’ The Study of Second Language Acquisition or Doughty and Long’s Handbook of Second Language Acquisition. They are both massive tomes and can double up as doorstops when not being read. Cook, Lightbown and Spada, and Mitchell and Myles are all available as Kindle eBooks.
Mitchell and Myles, Second Language Learning Theories
Gass and Selinker: Second Language Acquisition, an Introductory Course
Initially, I found this book quite accessible, then I started to get rather bogged down on the detail, especially in chapters like the one on Universal Grammar, where the detail and thick, academic language gets quite unreadable at times. For me, it’s more than just an introductory course. I suppose what I should have done is read a bit of each chapter, then go back to chapters where I wanted detail (which wouldn’t have included universal grammar!)
Vivian Cook, Second Language Learning and Language Teaching
Very readable book and refreshingly related to (and critical of) the real world of language teaching rather than the abstract world of academia. It’s probably the best introductory text of all.