The issue with most adaptations is to what you can adapt from the original source, and how to make it work. The obvious solution is to look at what you need, rather than what you want.
The first step is to look at the plot, and see what works as a book and what works as a comic. Although a comic is a literary work, it faces a lot of the same limits as film when it comes to literary adaptations. With film the source needs to edited severely, as only so much can be filmed, even with a full-adaptation. Look at it this way: A two-hour movie converts to about 300 pages of comic script. This means that a 200-page book of average complexity (think your average young adult book) should convert nicely to 300 pages. Obviously, you need to see if something can be dropped.
Start by shrinking the plot to a skeleton. Once you've done that, start stripping out threads that don't belong or don't really add anything to the plot. Try to limit yourself to four or five act breaks (events that signal that an act is ending and another is starting); this will not only help you eliminate huge swaths of the story, but will also help you focus the script a bit. Then start highlighting scenes that are important; you shouldn't need each and every scene in a book, so this should be easy. It may sound hard, but a lot of scenes in your average book are more flavor than substance; they are there just to develop characters or act as red herrings. If the scene doesn't work in one of your act breaks, then that's a good sign to get rid of it. If all it does is give history on a background character, get rid of it. If it just describes a character, get rid of it. Get rid of as much fat as you can.
At the same time, keep some of it in. If the scene is a fan favorite or allows the character a trademark moment, keep it in (not all of them, obviously; even though Tom Sawyer is known for its swimming scenes, you usually only see one or two in any movie). Just a like a good steak, some fat helps flavor it; just decide which fat is important for the right taste.
You should also cull characters. Don't touch the majority of the main characters; they are usually too important to the theme and hard to combine because of their importance to the story feeling right. By the same token, see what minor characters can be dropped or combined; as not all of them are important, but they do sometimes serve important purposes. It's important to see know which characters can be combined, and which are just chaff.
Once you have all of the events and all of the characters, you need to recreate the plot skeleton, but with only your chosen events listed. You also need to create a relationship chart between your chosen characters. Once you have those, you can then write your own script based on the original source.
Writing an adaptation is a lot more effort than it looks. But, if you truly love the source, then it can be a work of love. If you don't, then I hope you're getting paid well for it...
