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Sketching School (Learn as You Go)Sketching School (Learn as You Go) by Judy Martin
Buy used from: $0.28
An exploration of the varieties of sketches.  Hundreds of examples of professional artist's sketches done in many mediums.  Something to inspire everyone.
Figure sketching school (Learn as You Go)Figure sketching school (Learn as You Go) by Valerie Wiffen
Buy new: $17.94 / Used from: $0.74
Like its sister volume Sketching School, only this one focuses on the human figure.  Thinner volume and with more how-to, which isn't neccessarily a good thing.  Still, plenty of eye candy.
The Sketch Book for the ArtistThe Sketch Book for the Artist by Sarah Simblet
Buy used from: $8.50
A very attractive and well done volume.  Inspiring, thought provoking and certain to deepen your artistic thought process.  Highly recommended.
The ArtistÂ’s Sketchbook (Quarto Book)The ArtistÂ’s Sketchbook (Quarto Book) by Lucy Watson
Buy used from: $9.45
A good addition or substitute for the above Reader's Digest books.  Some basic ideas for beginners and a varied selection of sketches in a wide range of styles.
The Natural Way to Draw: A Working Plan for Art StudyThe Natural Way to Draw: A Working Plan for Art Study by Kimon Nicolaides
Buy new: $10.88 / Used from: $3.24
The mother of all sketching and drawing books.  Teaches you to teach yourself rather than provide limited formulas.  Get this!
Force: The Key to Capturing Life Through DrawingForce: The Key to Capturing Life Through Drawing by Mike Mattesi
Buy used from: $24.99
After mastering the drills in Nicolaides book, try this.  A perfect method for fast, on location skecthing.  Deepen and expand your vision with this book.
Drawing the Living FigureDrawing the Living Figure by Joseph Sheppard
Buy new: $10.36 / Used from: $5.45
An excellent and inexpensive reference.  Sketch first, then check and correct your work with the drawings in this book.
Fast Sketching TechniquesFast Sketching Techniques by David J. Rankin
Buy used from: $43.90
An easy method to get you started using a 9b pencil and some blending tools.  Nice instructions and drills to develop your skills.