I don't have easy access to the first fifty million digits of π, but I did manage to find the first million digits online without too much difficulty.
An ISBN-10 is a ten character long string that uniquely identifies a book. An example is "0-13-152414-3". The dashes are optional and exist mostly to make it easier for humans, just like the dashes in a phone number. The first character of an ISBN-10 indicate the language in which the book is published: 0 and 1 are for English, 2 is for French, and so on. The last character of the ISBN is a "check digit", which is supposed to help systems figure out if the ISBN is correct or not. It will catch many common types of errors, like swapping two characters in the ISBN: "0-13-125414-3" is invalid.
Here are the first one hundred digits of π:
3.141592653589793238462643383279502884197169399375To search for "potential (English) ISBN-10s", all one needs to do is search for every 0 or 1 in the first 999,990 digits of π (there is a "1" three digits from the end, but then there aren't enough digits left over to find a full ISBN, so we can stop early) and check to see if the ten digit sequence of characters starting with that 0 or 1 has a valid check digit at the end. The sequence "1415926535", highlighted in red, fails the test, because "5" is not the correct check digit; but the sequence "0781640628" highlighted in green is a potential ISBN.
There are approximately 200,000 zeros and ones in the first million digits of π, but "only" 18,273 of them appear at the beginning of a potential ISBN-10. Checking those 18,273 potentials against the WorldCat bibliographic database results in 1,168 valid ISBNs. The first one is at position 3,102: ISBN 0306803844, for the book The evolution of weapons and warfare by Trevor N. Dupuy. The last one is at position 996,919: ISBN 0415597234 for the book Exploring language assessment and testing : language in action by Anthony Green.
Here's the full dataset.