What Country Girl says is correct. Make sure that whewn you insert your hook into the last stitch you can see 2 vertical threads to the left of your hook and then continue as usual.
Any book about corcheting that covers the various techniques should have a chapter on Tunisian/afghan crochet. Also crochet stitch dictionaries. Some are better than others. Check your bookstores and then try to find the same thing at Amazon.com. You'll save tons of money that way. I would recommendDonna Kooler's Encyclopedia of Crochet as it has a very nice section on learning the technique itself for both left and righ-handed folks, as well as projects and a stitch gallery. The color illustrations are really great. Also a number of years ago Leisure Arts put out leaflet 2501 - An Afghan Stitch Sampler, which has 52 different stitches plus all sorts of opther info about the technique. You might be able to find it on the web.
I do a lot of Tunisian because it is kinder to the hands than regular knitting and crocheting. And the finished work looks so good! Glad to help if I can.