Getting Helen started on new books can be difficult, so it's a lot easier when she reads longer ones. She read Carole Satyamurti's retelling of the Mahabharata, which is 900 pages long and took her nearly three weeks, and then launched straight into Stephen Fry's Mythos, which kept her out of mischief for six days. And now she's started on Gustav Schwab's Gods and Heroes of Ancient Greece.
None of these are particularly difficult, but on the other hand none of them are written for children either. The Fry is modern and fairly colloquial, but the Schwab is a 1946 translation of an 1838 German work and the Satyamurti is, with its 27,000 lines, "the longest successful experiment in English narrative poetry in modern times", albeit a not terribly strict blank verse that strives for accessibility rather than linguistic pyrotechnics. (I bought the Satyamurti and Schwab for myself; Camilla bought the Fry for herself.)
This illustrates what I keep hearing from teachers — and which is obvious when you think about what constrains adult reading — that after a certain point reading comprehension is much more dependent on domain knowledge than on generic comprehension strategies. I don't think there are many areas where Helen can read like this, but she had previously read two retellings of the Mahabharata, by Namita Gokhale and R.K. Narayan, and a whole range of Greek myth retellings, notably Atticus the Storyteller and the D'Aulaires' Book of Greek Myths, so she was familiar with the plots and characters.
And Helen still loves her simpler books. She's looking forward to the latest Helen Peters book, A Deer Called Dotty, and even goes back to look at her flap books and simple early readers. I'm happy for her to read these, mostly because why not but also because they are a much more plausible model for her own writing.
Getting her hooked on a series helps, too. She's just read five books in a series of biographies by Short Books that started as Great Victorians (Queen Victoria, Brunel, Dickens, Lovelace, Nightingale, Livingstone) and has branched out into Great Lives (Austen, Davison, Mandela).