Don't know if anyone has suggested it here (I'd honestly be surprised if someone hasn't) but Principles of Mathematical Analysis by Walter Rudin, affectionatey

Don't know if anyone has suggested it here (I'd honestly be surprised if someone hasn't) but Principles of Mathematical Analysis by Walter Rudin, affectionatey known as "Baby Rudin" is a classic book. It's known for being relatively difficult and dense if you're just beginning with analysis, but if you go through the book and complete a fair number of exercises, it's an incredibly rewarding experience and definitely grants a ton of mathematical maturity.