I like Nalco's Handbook on water treatment. It's not online but it's a good investment and if you are using their products, you might be able to get a copy directly from them.
TD2K is right. I also found Nalco Handbook, quite useful at various occasions. In addition to that, I often refer two other excellent reference handbooks on the subject.
Betz Handbook of Industrial Water Conditioning, 9 e,1991,
ISBN 0-913641-00-6 and Handbook of Industrial Water Treatment by Drew Ameroid. I can check and let you know its ISBN later.