What is a good clean(no adult material), humorous, science fiction (preferably sci-fi adventure) book to read?

Please suggest a few books. I also like stories with romance but only if it is a small part of the story and very chaste (think original Star Wars movies). Please rate the books (G, PG, PG-13, R and why they are rated that way, such as for violence, thematic elements, etc.). Thanks for your... show more Please suggest a few books. I also like stories with romance but only if it is a small part of the story and very chaste (think original Star Wars movies). Please rate the books (G, PG, PG-13, R and why they are rated that way, such as for violence, thematic elements, etc.). Thanks for your suggestions

Here's a list of all the science fiction I've read (yes, I know, not very much): the Hitchhiker's Guide to the Galaxy (liked), the Stargate SG-1 Fandemonium novels (liked), Tatooine Ghost (liked), The Power of Un (liked), a few of the Stargate Atlantis Fandemonium novels (liked), Goners series (liked), Putting up Roots (liked), and The billion-dollar boy (liked).
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