This a good book for 800 - 850 rated player?

Chess Tactics for Champions: A step-by-step guide to using tactics and combinations the Polgar way

by Susan Polgar

There is a preview on Amazon

http://www.amazon.com/Chess-Tactics-Champions-step-step/dp/081293671X

I have a copy of this book and it is really good to get acquainted with basic tactical patterns, but as mentioned by NM ZaidejasChegis you need to use your brain to solve all problems. That's the only way to make any progress. 

In the end any tactical training, in which you use your brain to solve the problems (in stead of just guessing), will make you better at the game. Perhaps "a course in chess tactis", by Dejan Bojkov and Vladimir Georgiev is also a good choise. 

Get a copy of "Bobby Fischer teaches Chess" and go all the way through it. It's in workbook format and can be picked up used for a couple bucks on Ebay. Just be sure it hasn't been written in by previous owners.

Yes that is a good book for you. It will improve your tactics..

I have the book signed by her. She came to one of my uscf tournaments and was selling them for $15. I also like how there is a chess rating scale on the back.