This site has two parts. The first is a book that teaches chess tactics; you can read it by clicking anyplace in the table of contents below. The headings can be expanded one at a time by clicking on the [+] plus signs; or click here to expand all of them at once. The second part allows you to quiz yourself on positions from the book. Click the "Chess Quizzer" box to the lower left of this screen to try it. You can always come back here by clicking the "contents" button in the upper right corner of every page. If you came here by a different route, the easiest link to use to return later is www.chesstactics.org.
Hard copies of this site are now available: over 700 pages in total, with more than 1,000 illustrations and commentaries. Here are links to volume one and volume two. You can read a review here and a follow-up to it here (scroll down a bit upon arrival).