Many people will tell you life is too short to read books that aren't satisfying, but "bad" books are a great way to hone your self-editing and writing skills. When I was a kid and I didn't like the way a book developed, I would rewrite it in my head, and I'm willing to bet many of you did the same thing. Once we start writing our own manuscripts, we often stop playing with others' works. However, there's value in this old habit. Remember having to explain your opinions in school? Answering the question "Why don't (or do) you like it?" Analyzing your distaste will tell you what could be done better, and that will improve your own writing.
Maybe the books aren't even bad, but they fail to live up to your expectations in some way. Every so often, instead of laying the book aside, examine its failures. There's a series I'm reading right now that has an enticing plot and a well-developed world, but the author has some writing habits that disappoint me. I kept reading initially because of the plot, but then I found myself using it as editing/writing practice.
Here's how it works, with one example from that series:
1. Identify the flaw(s): The author has a tendency to state the emotions of other characters despite the first person POV. Ms. Author assigns them emotions through phrases like "...looked surprise," "...noted a look of affection," and "...saw her sadness."
2. Determine why this is a flaw to you: I have always preferred more subtle observations that clue the reader into emotions while possibly leaving the narrating character more uncertain. Assigning emotions in this way gives the narrator too much power. It leaves scenes lacking in emotional drama and therefore, a little flat.
3. Practice your skills: I rewrote small portions using characterization rather than emotion statements. (Rewrite in your head or on paper; doesn't matter, just try it!)
Keep in mind that this exercise is about the writing, not the plot. Save your plotting for your own manuscripts, and focus on what went wrong in this author's execution. I'm not suggesting you rewrite whole novels or invest hours in a book you don't love, but practicing like this on small sections can be really beneficial to your own work. So read some bad books, and if you really want to challenge yourself, try it with a book you love!