Well i agree that at least your DH ok'd the present..so that can shift some of the blame his way.
And if you aren't readers...how would you know what the book enclosed? I have been shocked by some of my choices at times..and I read a lot. Of course now you have me curious to the title of this book

...and every opinion is different when it comes to x-rated.
If you feel like apologizing then do...but DH should apologize as well...after all its his mom and he ok'd it too.
As for future gifts...send the pics of the kids...or a gift card for the book store ( and scrap a super cute holder for it!)
Someone said about people not acknowledging gifts (sorry i can't remember who posted) but i fear that common courtesy has disappeared. I have been in that same situation countless times with Dh's family ( i have actually caught some of them complaining about the gifts behind my back) That i finally told him this year it's his job to get gifts for them..and i won't craft cards either (except for the MIL who loves them

)
i have always looked at gift giving as a double joy ...the receiver should be joyful for the thought..and giving it should bring joy to the giver as well...and when it no longer is such that is when i quit. (makes me sad about thinking that way...but i hate being made feeling stupid or guilty about a gift too.)