I agree that you shouldn't wait until the announcement comes out. A lot of people don't do that anyway, and if you aren't close enough to be invited to the showers, you won't get the announcement either.
It also depends on the tradition of the family. For example, some traditions don't have baby showers, and only give gifts after the baby is here. Others don't want gifts too early - they want to be sure the pregnancy is viable. In both cases, there's the concern about jinxing the pregnancy or about the expectant parents having to get rid of things when a baby or pregnancy doesn't survive. Others like to start sharing in the excitement as soon as the pregnancy is announced, or at least when the first trimester symptoms abate and both people are more focused on the fun of a pregnancy.
I doubt they will buy books themselves - most people don't do that, as they are thinking more about immediate infant supplies and "necessities" than things an older baby will need. It's unlikely they will return books although it sounds like you want to be "first" but have provided gift receipts in case you are not.
Are you more friendly with the husband and that's your connection? Are you giving the gift to him, or mailing it to the home? Is he talking about it a lot and so you want to acknowledge it with a gift? It sounds like you have some reasons you aren't going to share, which is fine, so I'd just say to evaluate them and the relationship, and go from there.
I guess, barring any traditions that should be considered, I would wait until the 6-7 month point, which will be ahead of the showers but well into the pregnancy. At that point, people are thinking "baby" instead of just "fat and sick" (!) and will be appreciative and getting excited about gifts, but you will still be ahead of the frenzy of gifts, which it sounds like you want to do.