Economists these days use 'dollars' for 'US Dollars' with 'D' in lower case. Same for rupee, baht and renminbi, et al.
If there is no other dollar type currency in the book, it's ok to use $ sign before currency. Or so I think. $50 - no space between the sign and the number.
USD can be used in tables and figures.
When used alongside another currency, say rupees, then 'dollars' is preferable.
I guess then that I would use US$ when there are other currencies being mentioned in the book, say AU$.
Disclaimer: All my opinions. Use at your own peril! ;)
