Antimemetics Division is a fantastic read if you're at all a fan of the SCP Wiki -- being the author of some of the foundational entries in the wiki, like SCP-055 (https://scp-wiki.wikidot.com/scp-055), qntm shows a deep understanding and appreciation for the SCP universe and uses it to tell a compelling story about ideas, memory, and sacrifice.
It's worth noting that you can't currently buy a physical copy of this book anymore, but the original story is still available to read for free on the SCP wiki. https://scp-wiki.wikidot.com/antimemetics-division-hub
[light spoilers in this paragraph] This book caught me during a particularly difficult part of my life, when I was struggling with depression and self-doubt. Reading something that framed ideas as being things that are not only infectious and mind-altering, but killable was comforting for me, and gave me hope that I could climb out of the hole that I was in.
Ra, on the other hand, is a book set in a completely different universe, one in which magic not only exists, but is a bona fide scientific study, having been discovered in the 1970s. It starts out by exploring the ramifications of magic's use as self-defense, but evolves into mystery when the main character witnesses her mother performing magic that by all accounts should be impossible, compelling her to devote her life to uncovering the phenomena's true nature and origin.
This one is considerably longer, and paced slower than Antimemetics Division, but in my opinion is no less satisfying. The parameters by which magic works in-universe feel believable and self-consistent, and so do the reasons behind why some mages are able to bend them. The ending does feel slightly unsatisfying when the book is taken as a complete work, but when considering that Ra was originally released as a web serial over the course of years, I feel it's a bit more forgivable that the landing wasn't stuck perfectly.
What's particularly interesting to me about Ra is that after completion, the author hosted a Q&A thread on their site where people were able to ask questions about how certain mechanics of magic work in-universe, and the answers given by qntm show just how much thought and care they put into making magic feel less like a hand-waved deus ex-machina and more like a complete system.
Highly recommend both of these, as well as qntm's other works if you're looking for something that scratches a particular sci-fi itch.