Or that's my opinion anyway. But I know it's become more popular recently to end a book on a cliff hanger, especially in a series, so let's look at some of the pros and cons of ending a book on a cliff hanger.
It gets people to read the next book to find out what happens.
It provides a nice lead-in to the next book.
If there isn't a next book, a cliff hanger ending is one of the top ten best ways to troll your readers.
It's easier to write than an ending that wraps everything up.
Well, hey, it's SUSPENSE. Gotta love that!
...And, as I said, it's trendy, which makes it kind of cool.
I know it's been done in famous books by famous authors, I know it's fun to make your readers bang their heads against the wall, I know it's creative and angsty and hipster, but it's not nice. And if there isn't a good reason to do it (i.e. the story calls for it for some reason), it's just an attempt to be cool and/or troll readers, which is unprofessional.
I would say that ending a book on a cliff hanger is cheating, but since there isn't a rule that says you can't do it, it can't really be called cheating. It is, however, taking the easy way out because like I said above, it's easier to write an ending that's a cliff hanger. You don't have to tie up those loose ends. You don't have to find that sweet spot where the reader can walk away from the book with a satisfied feeling. You don't have to come up with an eloquent last line. All you have to do is stop.
It's also easier on you as the writer because it sells your next book. Of course your readers have to find out what happens, so the minute they finish the last page they hit the bookstores looking for the next installment. The trouble is you can write a cliff hanger ending without really trying, whereas it takes hard work to sell your second book simply on the strength of the world you've built and the characters you've created.
So what's wrong with easy?
Good sales aren't a substitute for quality writing. Your series may be popular for a few years but unless your books are truly well-written, they won't continue on as classics for years to come. Writing is art, and art should be beautiful. If writing books is what you do, you ought to write the best books that you possibly can.
BUT... Sometimes a cliff hanger ending can be truly artistic and exactly what your book needs. In that case, go for it. Just make sure you're not acting from baser motivations. Give your story the ending it deserves and the ending it needs.
*cue Dark Knight score*