From the perspective of someone who had watched the show prior to reading the novelization, I would have to say that the adaptation did the work not only justice, but surpasses its original. Though largely faithful when compared to the first and second seasons, there soon exists deviations that significantly affect both the feel and narrative by the start of Kumiko's second year. As is tradition by Kyoto Animation in their anime adaptations, 'Sound! Euphonium!' the anime, with no disrespect, takes the essence of the source material first and foremost, then incorporates the general plot-points as established within the story as their series directors seefit.

To the book series' credit, the story beats being augmented does provide those that are typically anime-only a 'reason' to revisit the franchise as depicted within its source. This is especially true for the aforementioned second year portion, wherein the utilization of multiple self-contained films as the medium to retell the entirety of that section left it much truncated and fragmented. I would say that the original version of the story was much better in its portrayal of the events, providing a generally more cohesive narrative that did not have to sacrifice much for the sake of runtime. Runtime itself is never a problem, given how some volumes were completely comprised of short stories, which were genuinely refreshing and provides some nice depth to the characters that would not translate over to the anime. The quality of the short stories heavily varied, but they were a nice addition nonetheless, and adds something the show sometimes lacks

In fact, I would go so far as to say that Movie 3 (Chikai no Finale) was a general disservice to the novels with how simplified and roughly paced everything ended up being when attempting to introduce an entire new group of cast members. 'Liz and the Blue Bird', meanwhile, coexists in such differing circumstances that I'd largely chalk the arc to personal preference given how director Naoka Yamada had done what she had done previously within 'A Silent Voice', which is to say rewrite the script to fit her auteur's vision. Up until this point, I would say the books and anime are, at the very least, on-par when compared side-by-side as a means to enjoy the story of Kitauji High School's concert band.

This is where the positives end.

Ayano Takeda's writing prose is generally quite poor, being too fixated on very specific and recurring details that quite often derails the story. Clearly written by someone still learning to be an author, certain parts were quite the slog to get through in the way she inconsistently paces the chapters. Despite what some on the internet might claim, it was not Kyoto Animation who attempted to make this story into a yuribait nothing-burger. The heavy removal of Shuiichi's role in the anime largely changed nothing in the grand scope of the story, as Shuiichi and Kumiko's budding relationship was itself barely a footnote in the books when compared to the love provided towards something as little as Reina's glistening, exposed nape.

Treading into minor spoiler territory, the endings between the book and show are highly deviated. While changing the source material is oftentimes considered highly controversial, I find that the decisions made by the anime staff lead to an all-around better conclusion to the story, and definitely provided an improvement in how Kumiko Oumae's arc flows into their shared epilogues. It feels almost like a fairy tale the way in which the main plot of the books ended, giving me the sense of a cop out that was being built up to that point.

Overall, still an enjoyable read that is worthwhile to those fans of the series, but absolutely holds tangible flaws that make it inconsistent.