Tight curves
Rocky;
As mentioned by others, a 9-3/4" radius is a pretty tight curve. I don't understand exactly what you mean by "hugging the outside" of a 12" deep bookcase. Model track radii are usually measured from the center of the ties. (Where the nail hole is.) To plan a curve take into account that you will need a minimum space of the nominal radius PLUS the full width of the track.(1/2 on each side of the curve). Add to that at least two full track widths for car overhang. To "hug" that 12" bookcase you would need a curve of 7" radius or so. That would still work with trolley cars, very short locos(0-40 steam or Plymouth diesel) and short ore cars.
You now say you're going to use the 12" radius recommended. Good step in the right direction.
How will a curve that's going to need 24" of layout width to make a U-turn, fit on a 12" deep bookcase?
I'm an N-scaler, and I used 12" As my minimum radius; until I bought one of Kato's beautiful
2-8-2 Mikado steam locomotives. Though it was advertised as being able to use an 11" radius, I found that in the real world, it derailed a lot. I ended up going to a 16" minimum radius. The point here is not to take the advertising as gospel, and if you must use tight curves, go with very short locos and cars.
Have you considered using Z-scale for your bookcase layout? The smaller Z-scale models could negotiate much tighter curve than N-scale. You might want to go to the Z-scale section of this forum and ask them about their minimum radius.
Is the one bookcase the only space available? My own layout was built according to an old article called "bookshelf railroads" in Model Railroader magazine. The sections are 16" deep and the trains run through them without turning around in that narrow space. My turn back curves are on wider sections of the railroad. The books sit on a shelf formed by the tops of the sections.
Just some other things to consider;
Good luck with whatever you choose.
Traction Fan
P.S. Here are photos of two of the straight, "run through" sections, and a turn back section.