When I'm writing something and I want to talk about a book I read years ago, I'm likely to go to some sort of summary to make sure I remember the book correctly. Nowadays, I usually go to Wikipedia for that. And yeah, when I use a source like that, my words are likely to be similar to the source. When we write, most of us refer to sources to ensure accuracy. If I'm quoting a passage, I note the source. But if I'm paraphrasing, I won't cite the source unless it's an original source.
Summaries of Moby Dick are available all over. I see no reason to cite them as sources.

-Joe-