OK, I admit I’m stupid. It’s not ebooks, it’s apps. I have just checked the websites of two of the most important museums in Europe, the Louvre in Paris and the Prado in Madrid, and both of them offer apps.
For our own “Kunsthistorisches Museum” in Vienna, a museum also in the top league, I found an app with “Stories”, but nothing complete. There is an ebook guide from the series “Rick Steves Tour” for €1.99 at Amazon though. At 30 pages it’s not comprehensive at all, but it has floor plans, some images of the most prominent works, and it might be enough to help along one’s memory a few months later.
Prague does not seem to have anything like that yet. That’s unfortunate, but I think in general we’re going in the right direction.