What's the etiquette on cross posting in several book groups?

New to Lemmy, wasn't on Reddit so i don't know the etiquette or how multiple groups are commonly used.

I'd really like to let everyone know about something (temporarily free sci-fi & fantasy ebooks and their writer) but don't want to spam or break norms. Is one post per group acceptable, even if I post to several groups or is that frowned upon?