The expanding universe by Markus Pössel; "An introduction to the physics and mathematics of the expanding universe, using no more than high-school level / undergraduate mathematics. "
Recommended Textbooks:
The best introductions are Relativity, Gravitation and Cosmology by Ta-Pei Cheng and
The Early Universe by Edward Kolb and Michael S. Turner
Edward R. Harrison, Cosmology, the Science of the Universe