There are several alternatives to string theory that have been proposed in the field of theoretical physics. Some of the prominent ones include:
Loop Quantum Gravity: Loop Quantum Gravity (LQG) is a theory that aims to reconcile general relativity with quantum mechanics. It quantizes space-time itself, and the fundamental entities in LQG are loops and spin networks. "Three Roads to Quantum Gravity" by Lee Smolin is a non-technical book that explores loop quantum gravity and its implications.
Causal Dynamical Triangulation: Causal Dynamical Triangulation (CDT) is an approach to quantum gravity that discretizes space-time into building blocks known as simplices. It focuses on the concept of causality and attempts to define a path integral over space-time geometries. While there are technical papers on CDT, there is no widely available non-technical book dedicated solely to this topic.
Emergent Gravity: Emergent Gravity is the idea that gravity is not a fundamental force but emerges as an effective description of a more fundamental theory. Various approaches, such as entropic gravity and the holographic principle, fall under this category. "The Universe in Zero Words" by Dana Mackenzie touches upon the concept of emergent gravity.
Quantum Einstein Gravity: Quantum Einstein Gravity (QEG) is an attempt to quantize gravity using methods similar to those employed in quantum field theory. While it has not yet reached the level of development as string theory, some researchers continue to explore QEG as a potential alternative.
Regarding non-technical books on alternative theories, it's important to note that the literature may be relatively limited compared to popular science books on string theory. However, some general books on the philosophy of science and the nature of scientific theories, such as "The Trouble with Physics" by Lee Smolin and "Farewell to Reality" by Jim Baggott, discuss both string theory and alternative approaches in a broader context.
Keep in mind that the field of theoretical physics is constantly evolving, and the status and development of alternative theories may change over time. It's always a good idea to refer to the latest research and publications in the field for the most up-to-date information.