[grogenaut]

What do you want, fiction? Non fiction, self help. Entertaining or thought providing or challenging?

Your question is insanely broad. Also what is the longest single work you have read? Many of the best reads take forever to get going. Also they are insanly long and thus intimidating

[oneplane]

Sometimes you can combine an answer to such a question by getting someone to read a relatively compact one like "The Physics of Star Trek". That won't work for people that don't like technology or science, but there are similar examples on other subjects. Works well for people who haven't started asking detailed questions yet, but may expect a fitting answer anyway.

If after starting or even finishing such a book and they like it, it gets a lot easier to recommend more specifically after that. If they don't like it, it either means the book needs to be less complex, or more single-topic and perhaps even non-fiction to remove more unknown elements.