About 200 of the 500 very rare first editions were sent to school libraries and most are still to be found.
But Dr Errington told CBBC Newsround Online there are specific things you need to check for if you think your book is a Potter first edition.
And of course it should be in good condition to be worth much.
- It MUST have the number sequence from 10 down to 1
- It MUST have the laminated pictorial board binding which is shiny-ish with no dustcover - not bound in cloth
- It MUST be published by Bloomsbury
If your book has all of these things, Dr Errington says the best thing to do is to phone a good auction house like Sotheby's or Christie's and talk to the kids' book experts.