This resource is for everyone, even though the title says it is for programmers. It also has questions to check your understanding.
http://www.wolfram.com/language/fast-introduction-for-programmers/
Of course there is a trade off, and this introduction may be too quick for your purposes. It is important not to lose sight of the big picture, which is possible in any given textbook. This video by Stephen Wolfram explains current thinking about Wolfram Language and suggests how to get started learning it.
For the past 12 years I have been an organizer of the Wolfram Science Summer School The students are supposed to learn Wolfram Language before they arrive, but it is a great place to get better, even though the main goal is to do a project.
