* Load https://en.wikipedia.org/wiki/Book:Solar_Eclipses into a book
* Visit https://en.wikipedia.org/wiki/Special:RenderBook/test and then when click "as HTML"
* Get "Could not generate the book" error
{F10148031}
Reducing the number of articles in the book using the book creator from 60 to 25 seems to allow the book to render.
This raises serious questions about whether our concatenation method will work at scale.
Note, that the limit seems to be on size of articles not number.
A similar book with 26 pages can be rendered:
https://en.wikipedia.beta.wmflabs.org/wiki/User:Jdlrobson/Books/The_big_book
Gergo has unanswered comments on the following patches (which maybe helpful for this task, but we should get to them in eventually):
1. https://gerrit.wikimedia.org/r/#/c/381277/
2. https://gerrit.wikimedia.org/r/#/c/381133/