I sold 640 copies of my book in one year
Well done. That's a respectable number of sales for your first year!The economics of self publishing is interesting. When I think back on
my first book I count at least 5 years research that enabled me to be
able to write it, 3 years writing it and then many years selling the
thing around industry.  By a pessimistic account I was about £500k
down :) Like you, the first year I sold about 500. Unlike online
fulfilment today, I spent that time stuffing envelopes, staring at a
slowly shrinking pile of boxes in the corner of my study, printing
labels, with occasional trips to the post-office. It was actually
work, and many times I thought 'this is crazy'. It doubled for two
more years until dealing with the orders actually became too much
(while doing a "real job").On the other hand, it got me a string of well paying jobs, established
me as the go-to consultant in a niche, invitations to talk and teach
all around the world, and basically transformed my career. So putting
a "net profit" number to it seems unfathomable. Each book you write,
if researched from scratch, is kinda equivalent to a PhD.What happened next is that I sold the title, from my own publishing
company to a large publisher (MIT). Publishers don't take many risks
these days, but they will buy a book that already has reasonable
number of sales as a going concern. The advantage is that all
mechanics and business is suddenly off your hands. Without an advance
to pay back you then have a modest income stream and time to write the
next book.Another effect of being a semi-professional writer, as a side gig, is
that other people want to pay you to edit, typeset, and review
works. Learning LaTeX and various markups and text processing tools
has been a huge gift. That can be quite fun if you're in a vibrant
field that genuinely interests you. And you don't have to be
associated with academia or a big-name institution for that to
work. In fact I'd say that academics these days never have time for
writing and that most other successful authors I know are independent.I hope your book takes off even more, but right now I think you can
count 600 sales in the first year as a definite success!