Given that it is not necessary to have an ISBN if you plan to self-publish an ebook only via Amazon (they use their ASIN), which is the best way to get one, if someone plans to sell his/her book also in epub (or pdf) format?
I know that there national official bodies which manage ISBN space: for example Bowker for the US and isbn.it for Italy; and there are "sublicensers" which are a bit cheaper. But there are also firms which give you a ISBN for your book, but remain officially the publisher for the book. Which are advantages and disadvantages for these options?