I have been reading this aloud with my 9-year-old son as part of our homeschooling curriculum, and it is actually painful to read because of how poorly it is written. The writing is clunky and awkward, and the sentences often continue endlessly, going on far longer than is necessary. I feel like this needed a much more thorough editing job than was provided.
Here's one example of a long and wandering sentence that could have easily been broken up into two or three separate sentences:
"It was one of four or five properties that Mr. Worth had bought and rented out, and of them all it was the least desirable, but since Mrs. Worth insisted on charging exactly the same rent for all the houses, whether larger or smaller, in good repair or ruinous, she had made the cost of living in the Tanner's Lane hovel far too high for the poor folk that might consider it."
And another sentence, just in the next paragraph:
"Mrs. Worth wanted no supper that evening but had me help her up the stairs to her bedroom, where at her direction I changed the bedclothes--there would be a monstrous heap of washing for me to do come Monday, I thought--and lit the candle, for she proposed to read in the Psalms until she fell asleep again."
We don't shy away from difficult novels; we just finished reading The Witch of Blackbird Pond, which has higher-level vocabulary than The Secret of the Sealed Room. I just won't waste time reading a badly written book aloud to my son for the next 3-and-a-half weeks.