The job of an optical lens designer is to design the lens system, either by designing a lens or multiple lenses into a system. Lens design Fundamentals (Rudolf Kingslake), Applied Optics and Optical Design Part 1 and Part 2 (A. E. Conrady), Modern Lens Design (Warren J. Smith), and Optical system design (Robert Fischer), Introduction to Lens Design: With Practical Zemax Examples (Joseph Geary), are all great books, just to name a few. Even in 3rd order aberration theory, taking the lens thicknesses to zero is thin lens 3rd order aberration theory in lens design.