Some of these were recommended to me by the Druid/Shaman I met and I would just like to share what he has with me with those who would wish to walk the same path as I and few others do. I will add on to the list as more books are recommended and/or that I have read and know for sure to be 'Must Reads'. I will also try to list authors so you can get the exact material that you need.

Note: I will try to list the books in difficulty ranges so you know what you should most likely tackle in the beginning, as you go, and so forth. Additionally I recommend picking up any book you feel may be imperative to your learning as a Druid (books such as, as an example, 'Dancing with Dragons,' 'Dragonlore,' 'Norse Magic,' 'Beginners Guide to Astral Projection,' by Edain McCoy, etc). And some of these aren't entirely in order or anything of how you should read them. Celtic Magic is a good starting point though and 21 Lessons of Merlin is a good second and the Mab being a good third stepping stone base foundation.

Initiate/Ovate Studies:
Celtic Magic by D.J. Conway
21 Lessons of Merlin: A Study in Druid Magic & Lore by Douglas Monroe
The Mabinogion
The Celts: A History by Peter Berresford Ellis

Optional for Beginners as well:
Dragonlore by Ashley Dekirk and Oberon Zell-Ravenheart
Dancing with Dragons by Conway
Norse Magic by D.J. Conway
Beginners Guide to Astral Projection by Edain McCoy
Enchantment of the Faerie Realm by Ted Andrews (I recommend his books rather highly)
The Rites of Odin by Ed Fitch (Not sure bout this one)
Celtic Women's Spirituality by McCoy
Crystal Balls and Crystal Bowls by Ted Andrews
How to Meet and Work With Spirit Guide by Ted Andrews
Contacting Your Spirit Guide by Sylvia Browne
Gargoyles: From the Archives of the Grey School of Wizardry by Susan Pesznecker
Grimoire for the Apprentice Wizard by Oberon Zell-Ravenheart
Companion for the Apprentice Wizard by Oberon Zell-Ravenheart

Intermediate:
A Druid's Herbal by Hopman
Crone's Book of Charms and Spells by Valerie Worth
By Oak, Ash, and Thorn by D.J. Conway
Animal Magick by Conway
Wicca The Complete Craft by Conway
The Druidry Handbook: Spiritual Practice Rooted in the Living Earth by John Michael

Optional:
Kindling the Celtic Spirit (I don't remember this one so well) by Mara Freeman

Challenge:
Advanced Celtic Shamanism
Animal-Wise by Ted Andrews
Animal Speak: The Spiritual & Magical Powers of Creatures Great & Small by Ted Andrews

Extra Reading that is Recommended and a part of Challenge:
The Golden Bough by Sir James George Frazer


Note: Remember to get out and also experience new things and sensations. These are also very important to your training as well as your 'Life Long' training.

Remember it takes time to learn as well as diligence, discipline, concentration, and more. But remember not to overdo it and be sure to take a time out when you need it. I recommend definitely reading 20-30 minutes a day as most schools will recommend.

Good luck and have fun!

Remember most Druids know things by constant memorization so keep that mind in tip top shape and memorize, memorize. You never know when that information will not only be of use, but will indefinitely prove you are a true Druid.

~~Note: Please see the 'Outter Ring Connections' for more recommended reading from some of the other Druid related sites.