At any NPC labeled Crafting Trainer, outside all tradeskill instances except Ironforge's. You will be unable to see a tier's worth of books until you are in that tier. For example, you can't see Essentials 20-29 until you're 20, or Essentials 30-39 until you're 30. You also can't see books for other classes.
Advanced books are drops; try the broker for those.
Essentials 50-59 are available at the Golden SCepter in Maj'Dul (DoF Expansion) and levels 60-69 are available in KoS (but I'd have to look up the NPC, lol).
edited for spelling, silly me.
Edited, Jul 24th 2007 11:58pm by Calthine