For sng- Sng strategy by Moshman.
For mtt- Harington on Holdem.
For ring- Ryan Fees free ebook(search here or on google, im to lazy to do it for u. PNLH,NLTP.
agreed ^
Also, if you haven't read a NLHE book before, consider reading Phil Gordon's Little Green Book for one to start out with.
then >> Harrington On Holdem Vol 1 & 2 - Tournament Play
then >> Collin Moshmann's SNG Strategy
Others to consider for Tournament play (imo.... & ahem... my opinion is 'the' opinion.. imo, lol).
Kill Everyone! (there's also a newer version of this with a supplement by Elky)
Winning Poker Tournaments - One Hand At A Time (Jon Turner, Jon VanFleet, Eric Lynch - - PearlJammer, Apestyles, Rizen, put together by Matthew Hilger). This is the best NLHE MTT book on the market by far (imo). It is much more geared towards high buyin MTT play & not really suitable for micro limits at all. It's great to help develop one's thinking processes for playing the game.
There's also ALL kinds of excellent articles out there online with invaluable info.
Another suggestion, try going to your local library to see if you can order up the books thru an inter-library book loan. Then, if you really like it you can purchase it (my guess, you'll end up buying the HOH books).
GL!