I'm editing a book that mentions Liberian Dollars (LD) and U.S. Dollars (USD). Should I put a dollar sign in front of the number or not? ($16 LD or 16 LD? $10 USD or 10 USD?)
In cases where adherence to the standard is important, you should not use the currency symbol if you use the official ISO 4217 code for a currency:
However, since many places only use those codes rarely, it can be wise to take the belt-and-braces approach is you don't have to follow ISO 4217 fully:
Other abbreviations depend much more on what would be understood in the context. Since LD$ and L$ are sometimes used for the Liberian Dollar, and US$ for the American, and since these forms are distinct, then I would recommend those:
Though if the book is about currency or finance matters, I'd go back to my first example, and use the International standard, avoiding local symbols entirely.