I tried several items in the Quick Search.  I was occasionally surprised by the search results as the books initially appeared to be unrelated but the search terms were always listed somewhere in the "Bibliographic Record".

The advanced search is slightly puzzling. 

Subject: As expected, this appears to search the subject fields of the Bibliographic Record.  For example, "Standard Oil" appeared in the subject lines of all four books.  One of the books (17090) includes the contents in the Title field (see Screenshot_2020-01-17 Project Gutenberg.png). [After mentioning this I've noticed that the same issue exists with many of the books.]

Title: Oddly book 17090 was included in the title results despite neither word actually appears in the title.  Probably due to the title + contents conflation previously mentioned.  And the third book (54399) was unexpected but the words did appear in the title of the book, just not in that order. 

So it immediately occurred to me to search for "Standard Oil" with the quotes.  "No record found. Please retry." I expected to see 60692 in the results. But the Help screen does say avoid punctuation characters so it probably works as designed.

I noticed that the tab results always says "Search on Titles" (see Capture.png) even though the selected search did not include a search item in the Title field.  It could be difficult to decide which should appear so I suggest use the first field from the top. For example, if an Author search term is included then always have the title say "Search on Authors".

Cheers,

Rick