Yeah, unless you're using a tripod and so shutter speed isn't a factor don't worry about "optimum ISO," just worry about what's good enough at a given print size. That camera book offers that information under the assumption you have a cheaper (point and shoot) camera, with a smaller sensor, fewer pixels, and more noise.
You will be amazed how good the 5D is; up to at least 800ISO it will be clean enough that noise isn't an issue for prints up to 16X20 is my guess, especially with a little noise reduction in light room. I recently visited with a semi-famous professional who needs fast shutter speeds above all else for his variety of work, and he'll go up to 3200ISO if needed with the intention of some images then being wall-sized prints.
If you're doing architectural photography and want the absolute best results, look into a tilt/shift lens, which should be very familiar to you if you used to shoot and process large format. In which case you would want to use a tripod, in which case shutter speed is a non-issue and so you could use 100ISO under virtually any light. HDR and exposure masking are also really popular with digital architectural photography, so that's another reason to use a tripod and bracket exposures.
But I think you'll be very surprised how good the 5D is across a wide range of sensitivities, especially when considering the printed image or final web output (not the camera original blown up to 100%, where noise is more visible). That book was written with cheaper, less flexible cameras in mind. The same way every lens has one "best" f-stop but it's not the only one you'd use, every camera's "best" ISO is good to know but nothing to get so concerned about using all the time.