A Problem with Series

(Note: I tried posting this last Monday, and it never appeared. So I'm re-posting it today. I apologize to my three loyal readers who looked for a post on Monday and were sorely disappointed. Now back to the regularly scheduled program.)

I just finished reading the second book of a series. In an effort to not offend the author or his fans, I won't mention his name, but I had a major problem with the ending of the second book. I'd like to preface this comment with the fact that I enjoyed the first book quite a bit. It went in a strange direction, but it worked for me, mostly because I loved the protagonist. So when I got book two from the library, I was excited to see what new adventures awaited this fabulous heroine. The second book was just as exciting as the first. The heroine was still lovable. The obstacles thrown in her path were deliciously horrific.

Then I got to the end. It was unbelievable. Unsatisfying. Illogical. And incredibly confusing. I closed the book and wondered what on earth had happened. I wasn't even sure she came out on top. Did she get what she wanted? Did she save the world? Did she actually accomplish something worthwhile?

I'm going to give this author another chance and read book three. But it got me thinking about my own novels within a series. Did I play fair with the reader? Are the endings satisfactory and understandable? Or do they leave the reader confused and frustrated?

Maybe the only way to know for certain is to ask my beta readers, as I'm entirely too close to my own novels. They work, in my mind, as I'm sure the ending in book two worked for this other author.
Does anyone out there have another solution to this problem?

-Sonja