I see no problem. It's done in many fields.
What I would probably do is offer a slight discount to "early birds". Take their orders, ship when available.
As long as they know it will not be instantly fulfilled. Although a backorder notice from a catalog is legal and accepted as well, and what's the difference.
One thing... I don't know who you're publishing it with, but it's very possible the thing is available for your order sooner than it gets on amazon
Check it out
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