I'm sure this topic has been raised here multiple times since the bobby is pretty mature at this stage. I bet every possible topic has been covered. But it's always good to review so I thought I would post my recent roam through the books on model train signaling that are out there. My layout is far from needing signals. But I got sidetracked and I like the topic so I just dug into it for fun.
The dominant author of books on prototype railroad signaling is Brian Solomon. Even his "basic" book is pretty deep, titled "Railroad Signaling". It goes through the evolution of signals and signaling systems. A lot of chapters stand on their own like "Towers and Interlocking" and "manual Block". But the chapters are stung together in a very logical way and he of course ends up with modern, computer-based signaling systems. It's about 160 pages and well worth a read if you like to read and are interested in signaling.
Brian's other big book is "Classic Railroad Signals--Semaphores, Searchlights and Towers". The level of detail here is striking, going all the way through stuff like "Rule 299--Track Pan Lights" for those of you picking up water on the fly! But the emphasis is on CLASSIC signals and he doesn't go beyond CTC as far as signaling technology. So for a lot of us, modeling the 40's, 50's and 60's, this book it right on the mark. The book also has enough prototype detail to satisfy most modeling needs.
Finally, I also picked up "DIY Advanced Model Railroad Signaling Electronics" by J. Richard Kerr, PhD. Kerr (published in 2015) has model train signaling down pretty well. And he doesn't hesitate to recommend the various makers of this stuff or to criticize them. He has comments in there on Azatrax MRD1, Bakatronics, Integrated and even Lionel detectors (which he panned). But he also shows how to DIY sensors to interfece with the MRD1s and in the end he gives you a complete home brew signaling system with a number of options. The primary context is HO DCC but he comments on using the stuff for 3-rail layouts and the differences from DCC for the 3-rail modeler are not significant.