Mathematical texts are hard to understand. The content of the work is always difficult, but usually the way it's written is also far from perfect. Obviously, readability depends on the level of knowledge and on personal qualities of the reader, but some texts still feel substantially better than others. Here are some works which I consider well-written.

Topology from the Differential Viewpoint by John Milnor

Characteristic Classes by John Milnor and James Stasheff is still the only mathematical book I've read almost completely. The appendix on algebraic topology is great for understanding the details of basic theorems like Poincare duality.

Milnor K-theory is the Simplest Part of Algebraic K-Theory, a paper by Burt Totaro. It remains the most understandable paper in algebraic geometry I've ever seen.

Lectures on Generic Vanishing Theorem by Christopher Schnell.