
FIY, I’m aware this is based on a light novel called One Piece: Episode Ace, which i own, but having still to get around to that, i’ll judge the manga adaptation as is.
In a way, this is one of the most OBVIOUS choices anyone could make for an One Piece spin-off, as Portgas/guese D. Ace is one of the most beloved character of the series outside of the main Straw Hat crew, had been for decades and then became even more popular by being allowed the non-insignificant feat of dying outside of a flashback.
Yeah, there’s no getting around that, nor there’s any point trying to cover it up, i distinctly remember people going around cons (even here in Italy) spoiling his final fate the very same year it happened, and his martyrdom raised its popularity to the very stars itself, as expected, and his very presence in the manga leaves many holes to fill in, so yeah, an Ace spin-off about his yet untold adventures is a fan pleaser ripe to make so much berries was just bound to be made eventually.
And since Oda liked Boichi’s take on the Zoro VS Mihawk (and then later the Nami VS Kalifa battle) we’ll get to that next, he asked Boichi to illustrate the whole manga adaptation.
Seriously though, you’re gonna get spoilers up the ass for a lot of the series, so if you are not up to date until the Marineford/Paramount War arc finale…. CATCH UP.
Again, SPOILERS AHOY big time, warned ya.

Curiously, unlike other light novel based on the series (or most of the derivate works based on the series), the light novel this manga version is based is actually considered canonical, so indeed is Episode A, and it’s not exactly a hard pill to swallow or anything, since Oda not only approved it as canonical, but also collaborated by giving the writers of the novel some ideas to use freely.
Plus it’s actually quite good, it fits perfectly with the established lore, it feels like you could actually attach it to the main series and not have anyone notice it’s expanded lore/spin-off stuff, and it scratches the itches of reading new stuff about Ace’s past adventures without re-treading too much the established events (which can’t be altered), it manages to balance out the desires to see some important moments again while giving new material that “harmonizes” with it, and Ace personality is what it should be, his mostly carefree attitude perfectly captured, plus the new character of Masked Deuce, more rational, knowledgeable and precise, plays as a good support to Ace’s bold and often risky life style.
In this case, the story is told by a guy stranded on a island (later naming himself Masked Deuce, as mentioned before) that Ace meets in one of his early adventures (including how Ace got his Devil Fruit powers) and becomes his second-in-command, as they form a friendship and eventually establish a full crew with the Spade Pirates, until they meet Whitebeard and Ace, after many clashes and troubles, becomes the 2nd Division Commander for the Whitebeard Pirates’ crew.

Even the art by Boichi is amazing, and offers an interesting prospect, as his style applied to Oda’s design might not be everyone’s cup of tea, but it’s also quite strong and different some might actually prefer his pulp artstyle for action and intense character moments over Oda’s.
Only gripe is that his artstyle can feel a bit jarring or odd as the tone change drastically over a few scenes, as in where Oda mastered the ability to transaction to comedy and drama and back, but this is a nitpick, it is, overall.
Maybe One Piece Episode A/Ace is the most obvious (and safe) One Piece crowd-pleasing spin-off ever, but it’s done quite well, i feel as good as this kind of spin-off/derivative works can get.