I really love the ideas in this book.  You can do lots of the things in it using your needle felting machine even though the book is written for hand needle felters.  
I particularly like that this book has lots of ideas for how to embellish ready-to-wear clothing.  At this price, this is a must have book.

Quantity
  Needle Felting Artful Fashion by Judy Jacobs and Kay Kaduce          $10.95  

                  

This book by Margo Duke is a must have.  Her book focuses on techniques using a needle felting machine.  She has the best techniques for combining textures and uses lots of different fabrics, fibers, and some other things that you might not have seen before.  The directions for each project are detailed and easy to follow.  I love this book.  If you own only one needle felting book, this is the one you should have.  It uses fabrics, fibers, ribbons threads and more.

Quantity
  Needle-Felting Magic by Margo Duke          $16.95
 

                  

If you are interested in using your needle punching machine to work exclusively with fabric (you will find no roving or similar products in this book), then buy this book.  Although not all of the projects use needle felting machines, most of them do.  Christopher also shows how to use combine free-motion embroidery with your needle punched fabrics.  This book is definitely eye candy for those who love decorator pillows.

Quantity
  Pillows by Christopher Nejman        $22.95

Although not a book that directly addresses the use of needle felting machines, I found that the techniques in this book really helped when I was working on making needle felted flowers.  Lynne's work starts with flat fabric (which you can make from roving using your needle felting machine.)  Then use her techniques for make a great variety of flowers.  If you are a flower lover and want to create flowers using your needle felting machine, this book will give you the background that you need to make the flowers look like you just went out and picked them yourself.

Quantity
  Fresh Felt Flowers by Lynne Farris     $24.95
 

I could not pass by this book.  This book is written primarily for wet felters, but the back half of the book is like a recipe book for creating different colors and patterns using roving.  Whenever I get an artistic block, this book is a fabulous block breaker.  It is filled with color ideas and many of the wet felted projects could easily be translated to creation by needle felting machines. 

  Creative Felting by Lizzie Houghton     $14.95

"Fast, Fun and Easy Needle Felting" by Lynne Farris is a book written for hand needle felters, but the projects translate very easily to machine needle felting.  It is definitely a beginners' book.  You will not find lots of the kinds of projects that you go GOO-GOO eyes over, but it provides a really good place to start.  The one thing that I especially like is that it shows how to blend roving to get colors that are more representative of what one might see in nature.   That was the main thing that I got from reading the book.  It was well worth the price for just that one very good explanation and the examples and diagrams it contains.  It basically shows you how you can "paint" with wool roving.

This was one of the first books that I bought about machine needle felting, yet, each time I look at it, I see something new,  Great book for a beginner.  Great book for someone looking for new ideas or just a spark of inspiration.

                  

I love this book.  It is full to overflowing with ideas and projects.  Lots of the projects use wet felting techniques, but they are very easy to convert the techniques to
machine needle felting.  Projects include ones for nuno felting, cobweb felting, carved felting, and the answers to most of the questions that beginners to fiber art need to know.
Color selection in this book is wonderful.  Finishing techniques are about the best that I have seem in any book.  Well worth the money.  This book is almost an
inch thick.