The 1st half of this review has NO spoilers and is intended for those who have not read the light novel yet.

(Like the novel, there will be spoilers for the original Death Note manga/anime)

The 2nd half will be marked as spoilers and is intended for those who have already finished the novel.

Disclaimer: I’ll refer to this novel as Another Note and the original manga/anime as Death Note.

When I was initially reading Another Note, I was wondering why it hadn’t been adapted to an anime yet. After all, I found it to be the perfect addition to the Death Note series.

It manages to perfectly match the fast pacing and smart deductive detective work (try saying that 5 times fast) of the original while also drawing upon various elements of the beloved story. From larger aspects like the titular Los Angeles BB Murder Cases that are referenced in Death Note, to the character of Naomi Misora, to minor characterization of Mello, to small but fun details like showing how L learned the martial arts technique he eventually uses when fighting Light -- it adds to lots of background elements of the original Death Note.

Naomi Misora stands as one of the highlights of the novel. Highly intelligent, harboring some past of her own that the novel alludes to, and just an overall badass, she serves as a great heroine with solid characterization in Another Note. Her interactions with L throughout the story are super fun and entertaining to see. The way they procedurally piece together the clues and close in on the killer is always clever, intricate, unexpected, and yet easy to follow and never drawn out. Plus getting to see Naomi Misora in action for an entire case adds a ton more weight to her death in Death Note.

But to return to my original thought, why hasn’t Another Note been adapted to an anime? It’s perhaps my favorite aspect of the story:
Narrated by Mello in the form of his notes, the novel takes full advantage of the fact that it’s in a written medium in a way unlike anything I’ve read before. It could never be adapted to a visual medium. (Unfortunately, I can’t really elaborate on this until the spoiler section.)

The novel draws upon the reader’s knowledge of the original Death Note, provides interesting insight into background elements of the original story, and still manages to function as its own exciting murder mystery. Another Note more than justifies its existence, captures the same feel as its source material, and ultimately fits right into the Death Note series.
I couldn't recommend Another Note more!

SPOILER SECTION:

Contrary to what others have said, there aren't really any major plot holes. This section is largely intended to clear some misconceptions I've seen.

Understanding the brilliance of Another Note requires an understanding of its main plot device:
Mello is an unreliable narrator. This is a narrative technique Another Note's author, Nisioisin, has used elsewhere in Monogatari and Zaregoto. It isn't an entirely alien concept, especially for Nisioisin.
The audience is not receiving the story as a first-hand telling of the events. Rather, it is being relayed to the reader through Mello in the format of his notes. He says in the first chapter that the most likely person to read the notes will be Near. Ultimately, it can be assumed that the diary in total is somewhat directed towards him. From Mello's characterization in the original story, it's evident that he's extremely cocky, very competitive, and always trying to prove he's better than Near. Another Note is essentially Mello flexing the knowledge he was given from L that Near never received. But he's not just saying he knows more and calling it a day. No, Mello turns it into a sly murder mystery for Near to try and deduce while reading through the notes.

Beyond Birthday never met L, so how could he adopt his persona and act like him?
He didn't. But you know who did meet L? The person who's narrating the story. Mello is trying to flex his greater connection with L, their role model, to Near. That is the purpose of Mello describing BB as assuming L's mannerisms (which doubles as a clever trick by the real life author to mislead the reader). In actuality, BB may have been weird in his own way but not as similar to L as Mello describes.

The state of whether Mello is alive or dead is perhaps the most vague. In the prologue, Mello says he "died like a dog" -- seemingly implying he's already dead at the time of writing Another Note. However, after saying that Near is the most likely to read his notes, he also states, "There's also a chance that Kira might read this... and I hope he does." This would seem to imply that Mello doesn't know Kira's identity (as he never refers to him as Light) nor has Kira been killed at this point. The most logical conclusion would be that he wrote these notes BECAUSE he was expecting to die in his final gambit to catch Kira. By the time anyone reads his notes, Mello expects to have long passed away. As I said before, though, it's open to interpretation.

Another criticism I've heard is that Ryuzaki leads Naomi through the clues too much and it doesn't feel like she gets to display her own intelligence. I'm not really sure what to say to this other than it's literally the point of the story? BB is always stringing Naomi along because they're HIS clues in the first place. Beyond Birthday tests Naomi and concludes that L has selected a worthy opponent for himself. He's found an opponent intelligent enough for him to lead along as Ryuzaki. It's made clear that Naomi's ability to parse Ryuzaki's hints is extremely impressive. What he didn't expect was for Naomi to figure out that he was Beyond Birthday -- acting as further testament to her intelligence.

Ultimately, I see Another Note as a surprisingly intricate murder mystery with a brilliant use of an unreliable narrator for its big twist.