When you answer with "Here's a book on this thing, buy it", what you're effectively doing is making a recommendation. That's fine, if I was trying to build Atomic Robo in Fate and there was an Atomic Robo Fate RPG sitting right there (there is), I'd appreciate someone mentioning it.
However, you should do what a good recommendation does:
You should be confident the product's any good and tell us it is. Have you read it? Does it have a stellar reputation among people who do what you're recommending it for? Does it do its thing well? This is that good subjective requirement where you assure us it's actually decent and isn't trash.
You should tell us about the product. What does it cover? Large scale battles, sure, but what can you tell us about that? This is the point where we realise that not only is it good, that yeah, it talks about exactly the kinds of things we want to do.
This isn't so much how Stack Exchange works as how making a decent recommendation to someone works when they don't have inherent trust in your recommendations.
If my best friend recommends that I see a movie, I'll very likely see it without asking more. If someone on the bus recommends that I see a movie, I'll probably want to ask more about it, or look it up on Rotten Tomatoes and other sites. You're the person on the bus, not the one whose judgement I place inherent trust in, so I'd want explanation.
Your recommendation might answer the question. I can't tell you whether it'll be a complete and acceptable answer, or whether it'll be a good answer, because that will depend case by case. But if you want to make a stellar answer, you should answer directly how large-scale battles work and provide a fairly complete answer on its own (if you've run multiple large-scale battles to great success, you should be able to talk about this) and recommend the book as further reading.