Ann Radcliffe was an English author in the Gothic style at the turn of the nineteenth century. Among the many interesting points in her remarkable career though, one little story is but poorly known. In 1796 she sent a friend a draft (copy here) of her latest novel to peruse. While his response is not preserved, we can perhaps learn something of it from the letter she sent him in reply (text here; image here; these (with the novel above) are all you need to solve the puzzle):
Can you follow Radcliffe's directions and find what exactly was so unexpected about this first draft of her novel? Surprising as it may have been initially, perhaps the punchline will be in fact far from unexpected by the time you find Radcliffe's message...
Edit: @GarethMcCaughan has pointed out that it is not desirable that a puzzle depend on a file contained on a site external to the stack.exchange or stack.imgur networks. For that reason, to satisfy any who may be concerned that the link to the novel may die, I have attached an image containing the entire novel. Each non-white pixel (reading left-right bottom to top) represents a single letter (the letters a-z, A-Z map straightforwardly to the 52 rainbow hex colours from #00008F to #920000, exact colours in the sequence a-z,A-Z here). This image is not necessary to use for the solution, as it contains the exact same information as the text file above, but were the text link to break, the text could be trivially recovered from the following image (the question already has a steganography tag).