A few Qs.
> WHAT IF A BOOK DOESN'T HAVE AN ISBN?
>
> If you have a book that doesn't have an ISBN, just make one up. It's that easy.
You don't validate ISBNs? Or you need us to invent a valid one that may one day be assigned and is currently unused?
> Unlimited Books Unlimited Users €29.99 per month
I am interested in this as my girlfriend's book collection is in the order of several thousand titles.
This price point just makes this a default no go. It would take her a lot of time and effort, serious investment, to catalogue all of her books.
The primary use-case is for insurance, and a secondary use-case would be to generate bibliographical entries for her papers and work.
Both scenarios are seldom-access scenarios, and 30 EUR per month seems very excessive.
You probably realise that your main competitor is the spreadsheet. Which has virtually no ongoing cost and can be backed up and shared easily, and Google Docs spreadsheet even means it can be managed online easily.
The only bit of value is to reduce the time to catalogue, the process of scanning the ISBN and resolving the book details from that.
Have you considered one-off pricing based on # of ISBN lookups and then a very low storage price... 30 EUR per 1,000 ISBN lookups, 10 EUR per year storage per 1,000 books.
And finally... is the ISBN database you're using a national one, or a global one? Of course ISBN is global, but some catalogues of data are country specific.