Two of them awere math books and the rest awere related to the brain.

You’re describing the books in the past, e.g. the action of buying the books. so you would use “were” even though the books are still existing into the present. E.g.: “the sky was clear yesterday” (past) “the sky is clear” (present)

I've been focusing on not just reading and forgetting the books but understanding and taking smart notes in order to review and recall what I read.

No need forgetting, but this was completely grammatically correct!

Two of them awere math books and the rest awere related to the brain.

We usually use past tense when referring to past actions despite still having the books.

I've been focusing on not just reading (and then accidentally forgetting about) the books but also on understanding andthem by taking smart notes in order to review and recall what I have read.

Two of them are math books, and the rest are related to the brain.

This sentence is technically fine, but since the previous sentence was in the past tense, it feels more natural to leave this sentence in past tense as well. "Two were math books, and the rest were related to the brain."

I've been focusing on not just reading and forgetting the books but understanding and taking smart notes in order to review and recall what I read.

In the first half of the sentence ("I've been focusing on not just reading and forgetting"), it's jarring to have "focusing" apply to "forgetting"-- it makes it sound like you're intending to forget what you read. I suggest rephrasing to move that clause so that the verb doesn't apply to it-- "I've been focusing on understanding and taking smart notes, in order to review and recall what I read, rather than just reading and forgetting."

Two of them are math books and the rest are related toabout the brain.

I've been focusing on not just reading and forgetting the books but also on understanding the books and taking smart notes in order to review and recall what I read.

I've been focusing on not just reading and forgetting the books butcontent of the books but also taking the time to understanding and takinge smart notes in order to review and recall what I read.

Two of them are math books, and the rest are related toabout the brain.

I've been focusing on not just reading and forgetting the books but also on understanding and taking smart notes in order to review and recall what I read.