Good old (available used for now) instruction books:
The book (this one's a reprint) is an overview in a way. Designed I guess for beginners, I still check it out for inspiration. If I'm being lazy about doing study sketches, cracking it open usually gets me in the mood.
Simple and straightforward.
There are also a few of these masterful drawings mixed in. Admittedly bearing little relation to a modern studio, it's a great drawing to study nonetheless--just look at it! I like the text, too. Guptill's words have been often quoted or re-mixed without credit. Lots of gems here if you take the time to read.
From Guptill's Rendering in Pen and Ink, still the all-time comprehensive pen and ink guide and overall drawing manual. This book is incredible--if you only owned one of the four listed here--this is the one!
I continue to learn new things from revisiting this book. Pretty much everything an artist needs to know about drawing and rendering (not just pen and ink) is included, from value to perspective to architecture to landscape.
In addition to instruction, there are sections containing pen and ink works by masters. Again, I recommend reading the text as well as looking at the pics--don't miss out.
Guptill manages to do it all for me with this volume--technique and inspiration!
I also have his book, Rendering in Pencil. Not being much of a graphite renderer, I don't find it as enlightening as the book previously discussed, but that's just my bias. The next book though, is indispensable if you love watercolor:
From Watercolor Painting Step-by-Step by Arthur L. Guptill.
There are a ton of expensive, slick-but-empty "how-to-paint-watercolor" books out there. Do yourself a favor--ignore most of them, find this one, and read it. You'll save some money and you won't find a better overall textbook for learning and re-discovering what watercolor's all about.
Maybe the thing I love best about Guptill is his profound knowledge yet refusal to be absolute. His writing style contains specific instruction without being dismissive of the many approaches an artist can take to make pictures. Only one example; this book allowed me to discover the use of white at a time when all my instructors were condemning that use as "not true watercolor".
I don't think any of these books are still in print, but used copies are still available on the internet. I have trouble not buying more copies of Rendering in Pen and Ink or Watercolor Painting Step-by-Step when I see them in used bookstores--have to talk to myself in order to pass them by.
Hope you found this helpful. Thanks for reading.