What are the best methods for independently publishing an academic book? I am a statistician and am thinking of writing a book on variations of regression.  I submitted it to one place who said that even if they accepted it, they would pay $1100.  That's very little. So I am thinking of self publishing but was wondering about the best methods - LaTeX? Pressbooks? or what?

"Best" in terms of ease of use and ability to produce the files needed for a book in various formats.
My advice is to go with a commercial publisher if you are so luck as to have one already willing to do an advance.  They have the marketing channels to do something with your book.  No promises of course, but self published works are incredibly unlikely to break through into being used.

Just take care of yourself in terms of rights and such.  (You need to give them the work in question of course, but make sure it does not impact any future production.)  Also, even while negotiating rights, realize they are "a customer" and really the most important one.  So don't be a jerk.  If anything try to establish a connection to be exploited later and give them a little "come on" (without making commitments) that you are up to publishing more with them.