I use Anki to learn material from textbooks, with great results. Creating the cards takes time, but so does summarizing the material (as Luke suggests); and the benefits associated to a SRS (specifically, the testing and spacing effects) make this approach clearly superior to any of the alternatives I tried.

Briefly, I write one note every paragraph, with a question on one side whose answer, written on the other side, is the main idea of that paragraph. Sometimes a note summarizes the content of more than one paragraph, if discussion of a single idea is spread over multiple paragraphs. Some textbooks include chapter or section summaries, and when they are sufficiently detailed, I'll rely on these to create the cards instead. Here's a sample card from Peter Gray's Psychology:

Front: How are ultimate explanations of behavior different from, but complementary to, proximate explanations?

Back: Ultimate explanations are functional explanations at the evolutionary level. Proximate explanations are explanations that deal not with function but with mechanism; they are statements of the immediate conditions, both inside and outside the animal, that bring on the behavior.

(This textbook is a particularly good example of "knowledge ready for ankification" because each page comes with 1-3 marginal "focus questions" intended to guide the student in the process of learning the material. So in this particular instance one can simply write these questions on the front side of each card, as I did.)

I also create a separate card for each term or expression that is explicitly defined and whose definition I don't already know. Many textbooks include a glossary, which simplifies the process of writing down these definitions. Here's a sample card, from Ivo Welch's Corporate Finance:

Front: What is annual percentage yield (APY)?

Back: APY is the simple rate of return, often simply called 'interest rate'. Banks sometimes use the expressions 'annual equivalent rate' (AER) and 'effective annual rate'.