This is the free online Plumb's Sixth Edition.
It's a little dated, but still has good info:
http://www.armchairpatriot.com/Home-Vet/Plumb's Veterinary Drug Handbook [Desk Ed.] 6th ed. - D. Plumb (Blackwell, 2008) WW.pdf

It's a little dated, but still has good info:
http://www.armchairpatriot.com/Home-Vet/Plumb's Veterinary Drug Handbook [Desk Ed.] 6th ed. - D. Plumb (Blackwell, 2008) WW.pdf