These are two of my favourite books on drawing. Both by Sarah Simblet. When I first saw Sketchbook for the Artist in the bookstore, I was floored. Her drawings and sketches are incredible. This book takes you through 90 different themes in drawing plus an overview of different materials and techniques. I kept going back to the bookstore to look at this book. Eventually I did buy it and I think for the first six months, I looked at it daily. I still go back to it for inspiration and advice on drawing.
The second book of Sarah Simblet is Botany for the Artist. Again, her drawings in this book are so wonderful to study. This is a solid crash course in botany and plant structures in order to understand the basic shapes that make up plants.
Sarah also has authored Anatomy for the artist. I don't have this one, but have looked through it in the bookstore. It is just as solid a reference as her other books.
Lately I have been looking through them again. Always good to go back and get a refresher. I'd be interested to hear what illustration books you go back to.