Vicki,
We do also own Clare Walker Leslie's
Keeping a Nature Journal. We found that one of hers to be a little overwhelming for first-time sketching (as hers are absolutely lovely) with not as many tips in the "how to sketch" area. Jim Arnosky's book takes you from the beginning of showing you how to appreciate nature, how to sketch and shade, and pointing out things to notice in how the sky looks or the shading in the water, etc. So, we prefer that one at our house!
The actual nature journal by Clare Walker Leslie that I originally linked to is mainly blank sketching pages with a very helpful Appendix on "how to sketch" basic leaf shapes, birds, twigs, etc. It works well with the Arnosky book in extending what was learned in there.
For younger kiddos the Crinkleroot books by Arnosky are very good too. My kids have used them for nature journaling by reading a two to four page spread once a week (on Fridays at our house) and then sketching something from the book in their nature journal. We have loved "Crinkleroot's Nature Almanac", "Crinkleroot's Guide to Walking in Wild Places", "Crinkleroot's Book of Animal Tracking", "Crinkleroot's Guide to Knowing the Birds" and "Crinkleroot's Guide to Knowing the Trees". The bummer is that these books are out-of-print, and so must be purchased used. They are in full color and do such a good job of inspiring a love of nature. A few other Arnosky books are still in print, but not done as well as the ones I listed above.
Blessings,
Carrie