Reddit mentions of Complete Accordion Method

Piano versus button matters mostly for practical reasons. Piano accordion will be easier to learn if you already know piano or another keyboard instrument, and is easier to find in the US. And once you make a choice, if you switch you'll have to relearn a lot. (Also, make sure you know the difference between chromatic button and diatonic button accordions which are a lot different.)

Yes, you can teach yourself, particularly if you have previous experience playing music. Getting used to it physically (wearing it properly and playing with good technique) might be better with a teacher, though I just read everything I could find on the subject. Like with piano, part of the challenge is playing different things in each hand at the same time, which takes practice no matter how you learn.

For sheet music, it's pretty similar to piano, but the left hand is different and somewhat easier to read. Also consider learning from videos. I have learned songs both ways and it exercises different skills, relying more on memory if you're learning from video. There are lessons on YouTube and Liberty Bellows has a good selection.

The Palmer-Hughes series is the usual recommendation for learning from sheet music. It starts from the very beginning and the difficulty increases very gradually as you go through the books. It's from the 1950's and so as you'd expect it only has older music.

Another choice that I like is "Complete Accordion Method", which ramps up more quickly and I like the songs better since I'm more into French music. (It also has fingering for button accordion, though that doesn't matter for me.) I bought the French version, but it is apparently available in English again:

https://www.amazon.com/Complete-Accordion-Method-Lucien-Galliano/dp/154003495X/ref=pd_sbs_14_1/140-8814933-8983643