Can a book be film noir? Well, given that "film" is in the genre title, I'd say not.
But, there are many film noir movies that are based on books.The Maltese Falcon and The Big Sleep, for instance, came from novels by Dashiell Hammett and Raymond Chandler respectively. Thus, it may be useful to categorize a book style as film noir. There are many kinds of detective stories: police procedural, cozies, analytical, and forensic, to name a few approaches. Film noir gives you an idea of what to expect.
Some of the defining aspects of the film noir cinematic style are visual-- tilted camera angles and dramatic lighting. That isn't readily captured in a novel... but it helped me choose my author photo.
One of the absolute requirements of film noir is sexual tension. I've kept that in mind while writing these. Many characters are attractive and there are complications in their interactions with each other that keep it interesting. First person narration means that, although the protagonist is himself a very attractive man, the descriptions focus more on the female characters.
Of course, film noir also dwells on the darker side of social behavior. Usually, it has to do with crime. Not all film noir stories are mysteries. Some are capers; some are escapes; some are thrillers. But at heart, there's generally a legal line being crossed or contemplated. Rafael has plenty of that to deal with, mostly from others, although he has to skirt the line himself in a few ways.
Finally, while this is not genre defining (film noir movies have been made since), the bulk of film noir movies were made from 1944-1954 in AMerica. Forever's Too Long is set in 1947, in New York City, so describing it as film noir (or just noir), helps imply the setting.
Film noir movies have an air of fatalism, pessimism and menace, though, and... The Forever Detective moves in and out of that. Rafael faces a heavy menace. There are hints that he's following a fate he knows nothing about, but others do. More will be explored about that in the sequels. As for pessimism, Rafael is torn between hope and fear. However, action helps him keep his focus emotionally and his warm personality and sense of humor contrast with the noir aspects.
So.... noir light? A dance/action number in shades of grey? Read it and decide.