I understand that this light novel was supposed to nothing more than an alternate timeline spinoff, but I still feel disappointed. The reason I feel disappointed is because this did not feel like it was Death Note at all, in virtually any capacity. This light novel was based off some sort of spinoff movie (that from what I understand, was not received very well), and I'm not sure how much the author of this novel (Named 'M') actually changed or if they were even allowed to. But the reality is that the only thing 'Death Note' about this novel was its use of the series' characters and world, and nothing else. This book can be boiled down to an extremely out of character action movie that really wanted to use the Death Note characters and its world, for some reason.
Right off the bat, the instance that this story takes place in is very disappointing. When you hear about an 'alternate timeline where L wins', your mind probably thinks of a lot of things. At what point did L win? How did L win? Unfortunately, you'll never know. It rarely touches on its derailment from the main series which is a let down, given that's one of the most exciting parts of the story. Instead, its about L attempting to save the world one last time, and focuses almost exclusively on that. The problem is that the basis isn't very exciting. There are terrorists trying to spread a virus globally and they're fighting L over the antidote so that they can have better control over it. Death Note is praised for its refined mind games that go into play, where intellectual battles between Light & L put both characters in intense situations. There is almost none of that here. It is written like an action movie. Jumps, stunts, physical encounters make up a majority of it. The little it does provide of the former are very shallow and fail to hold a candle to what the original Death Note offers.
Characters were a disappointment all around. The elephant in the room in this regard stems from L, to be honest. He is so unlike his usual self from the series. He keeps his intellectual traits, but the author doubles down on his 'sweets' side (Which, ok, was an actual thing but imo not to the extent it was exaggerated to in this story). Additionally, as mentioned before, he becomes an action movie hero which is so insanely different from his usual identity and did not make sense whatsoever. It was honestly depressing seeing the character treated that way. The newer novel-exclusive characters are otherwise very shallow and unremarkable. Characters were definitely a weak point for the book when you consider its just an out of character L and 3 shallow newbies.
I generally wouldn't recommend this book, but given that most people seem to like it for some reason, your experience may vary. It is undoubtedly very different from what Death Note embodies and it's up to whether you appreciate that change or not. I guess if you like that more straightforward action-esque kind of environment you may appreciate it.