When i say deep, i'm talking about certain mathematical concepts that most of us already know, but we dont really understand why we do that (ie math intuition) For ex. as stated in betterexplained.com, that the equal sign (=) does not always mean "is" or "equal to the value of" but it also means "with the same permanent relationship," "same destination" (he used Euler's formula to prove this), "refers to the same concept as", and " simplifies to"
PS. i am aware that Betterexplained.com has two books on math. Any other suggestions that are like them?