I wonder about the choice. Why Children of the Sky? It almost seems anti-speech. "The most powerful Nords cannot speak without causing destruction. They must go gagged, and communicate through a sign language and through scribing runes." Another thing to consider is the availability of the book. I don't think Children... is too bad from that point of view - you wouldn't want to pick one that's available everywhere.
I don't think providing a fifth book for Speechcraft would be too unbalancing. I don't imagine Speechcraft would be a skill that too many mods would mess with, but I could be wrong. I think The Lost Spires provides robes that boost speechcraft, but that's all I'm aware of. Mind you, I'm not a major mod runner
It would be nice if a book was designated as a Speechcraft skill book that it be placed in an area that wouldn't make it stolen for everybody. In other words, one of the guildhalls or something like that. Three out of the four Speechcraft books would be stolen if you actually wanted to add them to your collection. You're certainly tresspassing if you just want to read them and one wouldn't be accessible to anybody that wasn't in the thieves guild.
Perhaps this is why it was considered an oversight that needed fixing.