Drawing is fun and occasionally useful, while Inkscape is a nice and libre vector graphics editing program. Vector graphics are nice and scalable, too. So I've decided to learn the basics.
Firstly, there's a nice book, which is linked from inkscape.org. And the Learning Inkscape section. But they are mostly about getting familiar with the program.
Then it's time to practice, and that's what I have found to be rather difficult: without any drawing background, it's pretty hard to draw something that you wouldn't want to delete at once. But here is a solution: regular drawing tutorials, like on WikiHow's Category:Drawing. E.g., my first drawing in Inkscape:
And practice should make further ones better.
In addition to those tutorials, it might be useful to read something about drawing in general. For instance, there are anatomy books for artists, and plenty of other books.
And here's my third drawing, RMS:
"You Can Draw in 30 Days: The Fun, Easy Way to Learn to Draw in One Month or Less" appears to be a nice introductory book, suitable for learning with Inkscape instead of a pencil. Though it also feels oversimplified (e.g., drawing in perspective before even introducing it, and simplifying/not explaining it), and some would suggest to learn with paper and pencils first. A picture after its third lesson:
For drawing in perspective, wikihow and wikipedia may be helpful. There's also a nice Perspective Drawing Handbook (Dover Art Instruction) with a lot of examples/illustrations/explanations (I enjoyed reading it).
And here is a drawing in (quite possibly messed up) perspective:
