Track Plans
Track planning is the most addictive element of the hobby. They prescribe all the work to eventually take place in the real world, act as puzzles most inextricably and creatively solved, and represent the infinite possibility of what we could do with our space; with such being the case, it is clear to understand why all modelers are enticed by the thought of a track plan.
Most books of track plans from reputable publishers will always be high-quality, but the best always come with descriptions of each plan, noting how they might be constructed and operated. My particular favorites are 48 Top-notch Track Plans and 101 More Track Plans for Model Railroaders.
Much like HO Railroad from Start to Finish, 48 Top-notch Track Plans was a book I had very early in life and which I paged through constantly. It wins many points for having very extensive treatises accompanying each plan, describing the design constraints and considerations, and the construction goals and techniques. Unlike its predecessor, 101 More Track Plans also has short accompanying blurbs of each plan, but is especially notable for having some of the most creative, rare, intriguing, and thought-provoking plans I have ever seen.
Honorable mentions include Small, Smart, and Practical Track Plans, which elucidates the art of fitting satisfying model railroads into dissatisfying spaces, and 45 Original Track Plans, which demonstrates some very creative takes on layout subject and good examples of modern planning technique.