Looking of book on iron railing construction

There isnt a whole lot about this in print. First, go to the Nomma site- is the organisation of ornamental iron shops. Their magazine has a lot about this, and it is about 25 bucks a year. They also have some brochures and videos they sell - one of these might answer your questions.
Bending lambs tongues in railing cap, or bending pipe into returns? In either case, it isnt something you can do with no tools and no experience. I would recommend just buying premade bends- RBWagner makes all this stuff, aluminum and steel, and stainless pipe, in all kinds of sizes and radius. and their retail arm, JG Braun, at has some of the trickier, more ornamental components. also check Julius Blum- Most shops that install this stuff are just that- installers, and they buy it all premade.
Attaching parts is done in a variety of ways- Mechanically, with bolts and rivets. Welding. Heated and forged tenons, collars, and wedges.
Bending- well I bend stuff like this all the time, but I use a wide array of equipment, and it all cost money- Some stuff, I heat up in a gas forge, then bend by hand, hammer and anvil, or with a hossfeld bender- dies are available for cap rail, pipe, flat bar the hard way,etc. Some things I do cold with a hossfeld bender, or with a 50 ton hydraulic press, or with a powered angle rolls. Some things we forge from scratch using a power hammer, and dedicated tooling. Sometimes I rotohammer holes in the concrete floor, bolt stuff down, and use a hydraulic ram to bend.