With Christianity, it was one of the two religions of late twentieth century Earth referred to in the series, though the references were fewer.
The chief executive of the Thaarn in Dawn of the Gods called himself 'Caliph', a title used in Islam to refer to the successor of Mohammed, the person regarded as the leader of the entire Islamic community. There was no indication that this person had any Islamic connection; so the title was probably used as one of power.
In Bounty, the Amagons wore what looked like Arab dress.
In Gambit, two of the visitors to the Big Wheel were dressed as Arabs.
The Galaxy class cruiser Ortega was disabled by the destruction of its Ison crystal, which meant that it was effectively blind and could only travel at sublight speeds.