Lambda Calculus explained, with lots of neat tricks, and illustrated with Perl.
PIE prize! The whole IE roots list from the American Heritage Dictionary, legally and intelligently online! Yessss! Now to learn some grammar. . .
Vocabularies for the conlanger:
Let's start with the theoretical sets of semantic primitives and / or useful words. aUI claims to cover the universe in 31 phoneme/morphemes. A slightly larger philosophical language, Ro doesn't seem as elegant to me. Sona I've already linked, but its keen systematic radicals deserve note. Any practical, usable language must cover the ground spanned by Earth Minimal, too. Dublex's 400 roots may be pushing it a little, but they seem adapted for maximum derivational potential.
Moving right along, there are many shortish lists of words that one should consider covering. PIE roots fascinate me with how broadly each has drifted and mitosed over time. Examining modern derivatives of each, one can see that they have good glosses. Drawing from Speedwords should allow you to keep it real. Cover some names as well.
When you can manage anything said in Voice of America Special English (1500 words) you're on your way. And if your conlang covers, say, the Longman Defining Vocabulary, consider it all-growed-up.