As the title states, I’m curious how one would say “night reader.” As in, someone who enjoys reading late at night!
Seneca, in his 122nd letter, uses the words lucifuga "light-fleer" (< lux "light" + fugio "flee") and lychnobius "one-who-lives-by-candles" (< lychnos "lamp" + bios "life"). The latter seems to be his own invention, while the former appears a few other times in poetry in a more literal sense (e.g. Cupid disappearing from Psyche's bed before dawn).
[S. Papinius] erat ex hac turba lucifugarum. “Audio,” inquit [Pedo Albiovanus], “circa horam tertiam noctis flagellorum sonum. Quaero, quid faciat; dicitur rationes accipere. Audio circa horam sextam noctis clamorem concitatum; quaero, quid sit; dicitur vocem exercere. Quaero circa horam octavam noctis, quid sibi ille sonus rotarum velit; gestari dicitur. […] valde enim frugaliter vivebat; nihil consumebat nisi noctem. Itaque credendo dicentibus illum quibusdam avarum et sordidum vos,” inquit, “illum et lychnobium dicetis.”
Sextus Papinius came from that tribe of light-fleers. "Around the third hour of the night [9pm]," Pedo Albiovanus tells me, "I hear the sound of whips. I ask what's going on; I'm told he's reviewing the accounts. Around the sixth hour [midnight], I hear agitated shouting; I ask what it is, I'm told he's practicing his voice. Around the eighth hour [2am] I ask why there's the sound of wheels, they say he's driving around. In fact, he lived very frugally; he devoured nothing but the night. Thus, if the people who call him stingy and mean are to be believed," he says, "you'll also call him a person-who-lives-by-candles."
Lychnobius is also a pun in Greek: lichnobius would be someone who lives luxuriously, and most Latin-speakers pronounced the two identically. I have to thank Joel Derfner for bringing this wonderful passage to my attention, and I regret that I haven't been able to find a way to convey the pun in English.
If you're looking for a verb, there's also a very nice one: lucubrare means to read or write by lamplight. It's been borrowed into English as "lucubrate", one of those words that's listed in dictionaries but rarely if ever used in normal conversation.