Reddit reviews A Short History of Opera, Fourth Edition

As with all genres (and it seems like you're already on this path), I think listening is easily the most important part of the process. If at all possible, attend live performances - opera is very much a theatrical medium, and while videos can capture some of what goes on, it's just not quite the same as watching a live show. If that's impossible, I'd strongly recommend checking out the Met Live Broadcasts - it's probably the next best thing as they really try to capture the essence of the production.

You can also consider looking at books like A Short History of Opera or similar; while they're not explicitly about the process of writing opera, it's important to understand the genre, its history, and how it has evolved over time. Not to mention, that's probably the best way to quickly figure out which operas have stood out over time as remarkable, which will guide your listening and study.

Beyond that, the only reference I can specifically think of for opera would be the section in Rimsky-Korsakov's book on orchestration about vocal and choral writing. It is definitely the most comprehensive treatment of vocal and choral writing of any major orchestration text (and even then, it's only a couple of pages), and I find it pretty accurate.

I would also suggest speaking with actual opera singers (or opera students, at the very least), especially in terms of the technical aspects. Opera is extremely specialized in terms of technique so it's a good idea to get a feel for what the expectations and limitations are. Some subreddits to check out are /r/singing, /r/ClassicalSinger, or /r/opera.