I often recommend "The Druid Way" by Philip Carr-Gomm. You can reasonably find it on ebay or from any number of used book sellers. It is a comfortable read about a walk through the countryside however in the process it introduces many of the fundamentals and imparts a sense of the spirit of Druidry. Of course you can pick up some concepts from the material on our parent site: Druidry.org. The book you already have, "The Path of Druidry" by Penny Billington is an excellent choice as well. The other book you mentioned, "The Druidry Handbook" by John Michael Greer is fine also and does present a slightly different slant.
And....Don't forget to put the books down once in awhile and simply go for a walk in the woods or a park. There is much to be learned there as well.
