How do I recover a lost ISBN book number?

This is an interesting question because there are a number of things we need to consider when thinking about your answer. Specifically the International Standard Book Number (ISBN) system wasn't introduced until the early 1970s, so you're good in that department: your book wasn't published prior to that date, which would have meant you couldn't have had an ISBN.

On the other hand you say that you self published a book which is likely problematic: most self-published works are not assigned an ISBN number, so it's likely that you don't have an ISBN number in the first place.

Further, if you have a copy of the book (or the original manuscript) then I would imagine that somewhere in that book, either on the first page or even the back cover, you would have added your ISBN number or even had it appear as a bar code.

There are a number of different tools that let you try and search known databases of published works for a specific ISBN too, but let's start by checking out the official ISBN agency in the United States, the US ISBN Agency.

Unfortunately the US ISBN Agency only publishes its list of in-use ISBN numbers - over 150,000 of them - in print form. Very bizarre, but I cannot find an online searchable copy of the database.

There are other sites that have quite extensive databases of books, including many self-published titles, and can help you do more research to try and find your lost ISBN number. For example, biblio.com has a very sophisticated search form:

Note in particular that on the lower right corner you can specify date of publication.

Another great place you can search is Amazon.com, which has such an extensive database of books that it even has some listed with me as author that I never wrote!

Do a search on Amazon and you'll see a listing like this:

No ISBN shown here (though the first generation 10-digit ISBN is actually encoded into the URL that Amazon uses, if you want to be a real geek about it), but click through, scroll down just a little bit, and you'll find what you want:

Between those two - and carefully searching through a copy of your original self-published work - and you should be able to identify your ISBN number successfully.