The literature hints in the comments are already very good. When it comes to Sphere Packings, you can hardly get around Conway's & Sloane's book "Sphere packings, lattices and groups".
The following sources might serve as good complementary material:
"Sphere Packing, Lewis Carroll, and Reversi" by Martin Gardner. It published by Cambridge and AMS (which are known for very good books). It is not that heavy in terms of algebraic definitions and structures, hence it provides a really gentle introduction:
![enter image description here](https://cdn.statically.io/img/i.sstatic.net/bErdf.png)
"Lectures on Sphere Arrangements—the Discrete Geometric Side" by Károly Bezdek. It is more Geometry-orientated and thus a bit easier to read for people new to the topic. Along the fundamentals, Unit Sphere Packings (including proofs) the book also deals with Ball-Polyhedra and Spindle Convex Bodies, with Coverings by Cylinders, and with Conjectures such as the Kneser–Poulsen Conjecture and Alexander’s Conjecture:
![enter image description here](https://cdn.statically.io/img/i.sstatic.net/qzcDa.png)
"From Error-Correcting Codes Through Sphere Packings to Simple Groups" by Thomas M. Thompson. This book is a nice introduction to Sphere Packings from Group Theoretical point of view. It deals with Coding Theory, Leech Lattice and Simple Groups:
![enter image description here](https://cdn.statically.io/img/i.sstatic.net/JXrTv.png)
"Dense Sphere Packings: A Blueprint for Formal Proofs" by Thomas Hales. This is another good Cambridge book on Sphere Packings. It goes through geometrical topics such as Trigonometry, Volume, Hypermap, Fan, and Packing. It contains supporting visualizations, schematic illustrations and reads comfortably too:
![enter image description here](https://cdn.statically.io/img/i.sstatic.net/4HTuz.png)
Hopefully these additional literature hints can serve as additional help to dive into the topic of Sphere Packings.