Youtube - You can find a tutorial on pretty much any technique you want
clayartforums - defunct, but still a really great resource and source of interesting ideas
Digitalfire reference database - I've found it very useful for referencing some of the less common materials in ceramics.
Fara Shimbo's Crystalline Glazes and Chemistry for the Crystalieri - both go more in depth on the practice of making glazes and the chemistry behind them than most introductory books on the subject. Great, even if you have no interest in crystalline glaze.
John Britt's Complete Guide to High Fire Glazes - a great resource when first starting out making your own high fire glazes.
The Kiln Book - a great resource for both learning to fire kilns and learning to build them.
H H Stevenson's Ceramic Chemistry - it's antiquated, but it's also fascinating from a historical perspective and very approachable. I found it very helpful.
The Potter's Dictionary - it's expensive, but it's also replete with information. The sections on silica, alumina, expansion, viscosity, the colorants and the fluxes are all really well written.
From Mud to Music - you can find a lot of guides on making ocarina's and the like on youtube and similar sites. This consolidates ceramic instruments into a single resource with clear and easy guides. If you want to make clay instruments this should be on your shelf.
Glazes from Natural Sources - I haven't put this book into practical use much--there's a lot of hardware to process some materials you find around you. But, it is a great resource and if you're feeling industrious you can definitely make some unique, local ceramics without spending a fortune on materials. You may spend a fortune processing them, but at least you'll have dug them up for free..