Obligatory remark that "reading" a math textbook, and even moreso a basic one, is an active task; you can get the intuitions from youtube videos of the 3Blue1Brown variety, but to really learn you need to follow the text closely, put question marks on passages you're not 120% comfortable with, do the proofs that are left to the reader and the recommended exercises. If you're super comfortable with the material, you should do the more challenging exercises, usually marked with asterisks.

Basically you need a pencil, a lot of scratch paper and a capable waste basket; or a whiteboard all to yourself (but I get tired from being on my feet for hours on end).

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E: My litmus test is the Gershgorin circle theorem. Every so often I check if I can still prove it, and if I do I try to review the surrounding matrix. I will literally vote against hiring anyone who can't at least handwave why that's true.