A caricature of Douglas Adams is seem in the Guidebook's explanation about dolphin/human intelligence, sitting at his typewriter smoking a cigarette.
The line "to boldly split infinitives that no man had split before" is a poke at Star Trek (1966), whose catchphrase "to boldly go where no man has gone before" contains what many people consider to be a grammatical error - splitting an infinitive.
First appearance of Trillian's mice.