It's very hard when you see people parenting in a way that is different to how you would parent. It's also hard to bite your tongue, or know what, or when to say something.

Rather than buying books for your SIL, perhaps mention in conversation books or resources that you have found useful. I've mentioned the Science of Parenting, and Pinky McKay's books, the Wonder Weeks, and a few other "gentle" books in conversation with other parents, but in a non-direct way, iykwim? I've found that 99% of the time, most people are not interested in my views on parenting any more than I am interested in theirs (not speaking of BB ladies, of course), so trying to force-feed them books and information that is not welcome doesn't get me very far.

You could always mention how much you love BellyBelly and hope your SIL signs up...if you want her around, that is