Sure, “mom’s birthday”.

*wink wink, nudge nudge*

Quick question: why the fuck do you want to gift 20GB of erotic books to your mom???

https://bookshop.org/p/books/exit-to-eden-anne-rampling/0da5f5fc77b7f09e

Anne Rampling aka Ann Rice. 'Exit To Eden' Dominatrix falls for the latest pleasure boy hired for her island paradise.

https://bookshop.org/p/books/the-silver-metal-lover-tanith-lee/49390bedd75175ac

Tanith Lee. 'The Silver Metal Lover' In the future, an overbearing mother buys her pampered daughter the most advanced robot ever created.

Both authors have other titles Mom might enjoy.

Please check out my work. There are Kindles and audiobooks

Smart, Mean Girls:

Kindle: https://www.amazon.com/dp/B0GC7N9SBJ

Audiobooks: https://libro.fm/audiobooks/9798295379666

https://books.apple.com/us/audiobook/smart-mean-girls/id1866143684

https://play.google.com/store/audiobooks/details/Sarah_Smart_Mean_Girls?id=AQAAAEDqrwnv8M

Dominating Chloe:

Kindle: https://www.amazon.com/dp/B0GCKQ1J73

Audiobooks: https://libro.fm/audiobooks/9798295386367

https://play.google.com/store/audiobooks/details/Sarah_Dominating_Chloe?id=AQAAAEAa3EScTM

https://books.apple.com/us/audiobook/id1868170216

I wonder what the fuck you're going to get dad for his big day.

So I ADORE R. Lee Smith's "Heat".

Good luck mom.

What does she like in other sorts of fiction?

I am a mom & the Kushiel series, main character a sexual masochist, love those books so much. Not sure they are a reasonable thing to spring on someone unprepared but man are they good.

On the more typical romantasy stuff, those Empyreon books are very good. I also liked One Dark Window & its sequel.

In general erotica, I'm not so familiar with it. But that is the word to use if looking for stories with a lot of sex, stories about sex.

Is she into manga?

An archive of ao3.org?

sooo... somewhere between 10.000 and 40.000 books?

Not sure there are that many different ones in this category, even if you consider it's popularity.

Morning Glory Milking Farm

Not my field but what about the Outlander series?

If she is at all into fantasy, there are some nice and spicy romantasy series out there.

Get In My Swamp.

Yes, it is exactly what it sounds like.

There are sequels.

I like Rawnie Sabor, but most of their works are queer.

Any of the books written by Ali Hazelwood. Also look for Elle Kennedy, Piper Rayne, Laura Thalassa, Stephanie Archer, Laura Ferguson, Lisa Lang Blakeney, BK Borison, Brynne Weaver, Gina L Maxwell, Sara Cate, Tessa Bailey, Monty Jay, Julie Soto, Mariana Zapata (Queen of slowburn).

For fantasy romance try Keri Lake, Katie Robert, Rebecca Yarros, Callie Hart, Sarah J Maas, Sen Lin Yu, Rachel Gillig.

That should get you a lot of books

https://www.smashwords.com/books/search?query=Romance+erotica

The Boss by Abigail Barnett if she likes em REAL (but responsibly) spicy