I'd suggest two things are of value in this app: firstly, the the "format" of these particular editions, and, secondly, their curation as an app.
If you want to read "Excursions in Madeira and Porto Santo", by T. Edward Bowdich, Esq. (one of the books), then archive.org has an excellent copy, here:
However, you probably would never read such a book. You may, however, browse through the version included with the British Library app. The BL version is a very nice edition, with colour plates. More importantly, the BL version is presented as part of a collection of interesting old books, which means that a lot more people will be reading about Mr. Bowdich, Esq.'s adventures this weekend than would normally be doing so.
The presentation of the books is of general interest. By contrast, the actual text content of the book is of very specialised interest, and the (decontextualised) pictures not much less so.
The problem is that there are now a pile more mediums to deliver the message on depending on how much you can afford to pay out.
The information should be uniformly available to all, which I believe is the point of the British Library.
However, having been there a number of times, it's a pain in the arse to use the BL anyway and copying anything for reference (like you used to do down your local library) is virtually impossible now. So it's literally sit and copy the work out.
