If you are like me and self-study a lot of mathematics, but often find most textbooks completely impenetrable and difficult to read, here are some books I've found to be very clearly written and well-explained on some topics.

Some are more obscure, some are well-known. There are of course many others, but the below are the most friendly I've personally found for self-study and requiring minimal background knowledge.

• Number Theory: Elementary Number Theory & its Applications (Rosen)

• Real Analysis: Understanding Analysis (Abbott)

• Abstract Algebra/Group Theory: Contemporary Abstract Algebra (Gallian)

• Point-Set Topology: Topology (Munkres)

• Algebraic Topology: A First Course in Algebraic Topology (Kosniowski)

• Ring Theory: A First Course in Rings and Ideals (Burton)

• Module Theory: Modules: A Primer of Structure Theorems (Head)

A couple other more niche topics:

• Topological Data Analysis: Topological Data Analysis for Genomics & Evolution (Blumberg/Rabadan)

• Geometric Group Theory: Office Hours with a Geometric Group Theorist (Clay, Margalit, et al)

Hope some of these recommendations are helpful! Feel free to add more in the comments!!!

Edit: A couple more I thought of:

• Set Theory: Naive Set Theory (Halmos)

• Proofs & Problem-Solving: Book of Proof (Hammack)

• Linear Algebra: Linear Algebra Done Right (Axler)