What's the connection of a watery food to a book?

Question

Life and Letters of Charles Darwin, volume 2 includes the following letter from Darwin to W. Ogle, written with regard to Dr. Ogle's translation of Kerner's book, 'Flowers and their Unbidden Guests':

I have now read Kerner's book, which is better even than I anticipated. The translation seems to me as clear as daylight, and written in forcible and good familiar English. I am rather afraid that it is too good for the English public, which seems to like very washy food, unless it be administered by some one whose name is well-known, and then I suspect a good deal of the unintelligible is very pleasing to them. I hope to heaven that I may be wrong. Anyhow, you and Mrs. Ogle have done a right good service for Botanical Science. Yours very sincerely, CH. DARWIN.

What's the connection of a watery food to a book, and what's the difference if it was administered by a well-known person or not?

Answer 1

As indicated by Stuart F. in a comment, it seems likely that Darwin was making an allusion to the difficulty of the book and how that might translate to it having more substance and value. A young child, or an invalid, may be given watered-down food and drink to make it easier to digest, which will require a larger volume for the given nutritive value. Similarly, a concept can be delivered in a "watered-down" manner, with the same benefit and drawback.

The allusion to "one whose name is well-known" is referring to the literature, namely the idea that much of the public may prefer the "watered-down" text, but if the person who is writing it is well-known, they may do the extra work to try to digest the material in question, especially if it allows them to requote it to look smart, the unintelligibility there being useful because it others, hearing them, may not immediately understand, and therefore assume that the speaker must be intelligent if they understand it.