Before I taught composition the first time, I reviewed lots of composition textbooks. There were some that were OK, some that were fairly good, and some that weren't so great. But there were none that I felt were worth the ridiculously high prices that textbook companies were charging. Students need models to write well, but in the internet age, there are plenty of models available online for free. It did take more work on my part, because I had to build the course from scratch, but in the end, I had exactly the course I wanted (notwithstanding the fact that I didn't want to teach composition at all).
So I thought about what students really needed, and realized that most students want and need to build their vocabulary. And I had this fantastic, amazing dictionary that I randomly found at the U of Illinois bookstore (because I used to troll language dictionary sections in bookstores, before I found this dictionary) that was perfect for the advanced second language learner (beginners, too, but beginners can get by with the one that doesn't weight 10 lbs). This beautiful dictionary was the Collins Spanish-English unabridged dictionary, and it's perfect because it's the only dictionary I've found that includes word usages with the possible translations of a word. This is essential for second language learners, because if they're using a dictionary (or online translator/dictionary) that just gives them a list with no clues as to what the differences are between the different words, they will just pick one, and invariably, it will be wrong. The Collins dictionary gives examples, in phrases and sentences, in both languages, so that learners can tell if that's really the word they're looking for or not. For words with multiple meanings or parts of speech, this is crucial. So I made the dictionary the required textbook for my composition classes. The last time I taught composition, in spring of 2012, my entire class was online except the required dictionary, which students found odd, but at the time, there still wasn't an online dictionary that came close to the breadth and depth of the Collins dictionary. But thanks to my friend and colleague Gwyneth Cliver, a German professor at the University of Nebraska at Omaha, I learned today that the Collins dictionaries are now available online for free! I compared an entry for the word 'can' from my paper dictionary to the online entry, and I'm happy to report that the online version contains all of the same information as the paper version. So while I'm thankful that I don't have to teach composition again, if I did, there would now be no textbook requirement, since the world's best dictionary is available free online.
(I love this dictionary so much that I have multiple copies of the unabridged version. I justify this compulsion by saying that it's too heavy to cart back and forth between home and school, so I need a copy in both places.)