The European Library has an SRU interface, it’s just a bit hard to find. Tellico can use it, though the data is a bit sparse.

Host

www.theeuropeanlibrary.org

Port

80

Path

/sru/sru.pl

Format

Dublin Core (although MODS seems to work, too)

It doesn’t seem to support ISBN searches. And the default Tellico search must be interacting strangely with their collection index. Test searches such as “Harry Potter” don’t return anything like I would expect.