Am I overreacting or is my MIL overstepping boundaries?

I met my husband when my kids were 4 and 6. They lost their dad and only my oldest son remembers him. So him and my MIL have been in my kids lives for 10 years now and they call his mom Nana. We see his mom daily because we have not had a car for over 2 months because of a car accident and she's been driving the kids to and from school and taking them to their clubs. Last week she took my kids to get their Christmas presents from her because she knew she wasn't gonna have money around Christmas and just wanted to make sure they got their gifts. She likes to just take them to pick stuff out instead of trying to surprise them. My 14 year old son got some air pods and my 16 year old son who's a huge reader just asked to pick out a few books so she let him. I didn't think much of it until I got a chance to see the books he picked and one was that book written by Jeanette McCurdy that called I'm glad my mom died. I lost it and asked my son about this book and asked was he wanting me to die or something? He said no, he just saw a tiktok of her talking about a mom and was curious about her childhood. I made him take the book back up to Walmart but since we didn't have the receipt or the card that purchased it they couldn't accept it. I asked to speak to a manger and asked did they think it was okay to sell inappropriate books to children. She was telling me they don't really I.D for books and I spent a while arguing with her. I end up taking an Uber to see my Mil so she can return this book. She apologized and said she didn't look through the books he picked but she would find the receipt and return it. I asked why she wouldn't look through the books he was buying and this is really overstepping my boundaries. And did she know all the dark stuff that is in this book. She said no. She just let him pick the books out. I asked again why she wouldn't look through the books he was trying to get to make sure they were appropriate. She got an attitude and goes "Sorry! I thought 16 was more than capable of picking out a book to read. I didn't think I had to!" I told her I don't appreciate her sarcasm and then she starts trying to play the victim and said she used her on disability check to get my kids presents and she doesn't know what I want from her because she already apologized. I said I want her to know she's not the parent. She's not even there real grandparent. I think that hurt her feelings because she got really mad and asked if I don't see her as a grandparent why is she picking my kids up from school and clubs and getting up at 6am to get them to school and why is she doing anything for us at all. I said because I married her son and she said she's done helping me because all I do is nitpick everything she does when all she's tried to do was help us and love my kids. I told her that I don't care and we won't be having her over for Christmas. We left. She apparently called my husband in tears telling him what happened and he's taking HER side. He also said I had no right uninviting his mother to Christmas without telling him. I asked if he thought that book was appropriate and he said my son has seen worse in TV and he's not a baby and that either way I should have accepted her sorry and moved on. I think her not bothering to look through the books was irresponsible on her part and end of the day I'm the mother. I've asked my own family their opinions and my mom agrees with me but everyone else said I overreacted.

@H I haven't read the book but I do know it's dark.