honestly i don't think the textbooks available for thai are all that great since they're typically aimed at tourists, rely on romanization, and don't get that advanced especially compared to what's available for more popular languages. here are the ones that i used when i was just starting out, but i don't recommend you treat them like college textbooks and spend like sixth months going through it – just pick one and get some basic vocab and grammar down.
- Spoken World Thai
- teaches basic vocabulary and grammar over the course of 15 lessons. i recommend being able to read thai before starting this book, although there are romanizations. comes with culture notes, reading and dialogue practice, and comprehension questions. i've uploaded the audio here.
- Complete Thai
- teaches basic vocabulary and grammar over the course of 17 lessons. the above link goes to an archive.org copy where you can 'check out' the whole book for an hour at a time. the audio for this book can be found here for free; an anki deck for this book can be found here.
- Mango Languages
- interactive website and app. it's organized similar to a textbook with major units broken up into smaller chapters, but also has a vocab flashcard SRS system built into it. it was free through my local library; you can check if yours carries it through the link above.
- FSI Thai Language Course
- website that has taken the Foreign Service Institute's thai language course and converted it to text to make it more accessible to modern learners. there's a lot of audio and it's embedded right where it's needed, you can toggle IPA transcription on or off, and is much more thorough than other courses i've seen. however it was made in 1970 and is definitely showing its age, plus it was designed for 8 hours a day of in-person drilling and instruction so there's not much in the way of self study exercises.