If you set the correct FL and pixel size in your guider software, guiding error will be displayed in arcsevonds. Otherwise it will be displayed as pixels.
I'm not aware of any books, specific on guiding. Two general books on astrophotography are
Making Every Photon Count, by Steve Richards, and
Long Exposure Astrophotography, by Allan Hall.
A good online resource is Craig Stark's website:
www.stark-labs.com/craig/articles/articles.html
Stark is the original developer of PHD guiding software.
As for best guiding fl, a general rule of thumb is that the guiding pixel scale shouldn't be more than about 2 times the imaging pixel scale. In practice, if your guiding is too coarse, you can't get good pictures, while if it's too fine, you'll be guiding the noise.
Hope this helps.