This is a list of the giants in the field:
1- In terms of L2 acquisition: Stephen Krashen; Jim Cummins; Wayne Thomas and Virginia Collier; Nancy Cloud, Fred Genesee; Yvonne and David Freeman. Larry Ferlazzo is also a leading ESL practitioner.
2- In biliteracy and the emerging translanguaging movement, look at the work of Ofelia Garcia, Socorro Herrera, Sonia Soltero, Kathy Escamilla, and the team of Karen Beeman and Cheryl Urow.
3- Crystal, Canagarajah, Bailey, Cummins, Larsen-Freeman, Gass, Ur, Graddol, Nunan, and Celce-Murcia. Some of those are leading scholars for their research and scholarship, and are widely known through impactful publications.
Stephen Krashen*, a linguist and literacy researcher, he developed numerous hypotheses of language acquisition, significant work in literacy. Paulo Freire*, critical pedagogy, adult ESL and literacy.
If you need some book references that are fruitful in the field, I highly recommend:
1- Principles of Language Learning and Teaching. By H. Douglas Brown (fifth edition).
2- How languages are Learned. By Patsy M. Lightbown & Nina Spada (Fourth edition).
3- Teaching English as a Second Foreign Language. By Marriane Celce-MMurcia, Dona M. Brighton, & Marguerite Ann Snow (Fourth Edition).
4- Language Assessment: Principles and Classroom Practices. By H. Douglas Brown & Priyanvada Abyewickrama (Second Edition).
5- Foundations for Teaching English Language Learners: Research, Theory, Policy and Practice. By Wayne E. Wright (copy of 2010).
Best of luck in your TESOL journey.