Something like that happened to me three times in the last months. Once with the second book in a duo, once with the third book in an ongoing series and once with the fourth book in an ongoing series. Each time I was very disappointed with the last book of the series I read even though I enjoyed the books before.
In cases like the ones I just described, the first thing I do is to get rid of the "offending" book (give it to other readers). Then, instead of being frustrated and disappointed by the book and by how the series continued, I created a new ending for the whole series in my mind.
After I created my own ending I'm happy again and can think about the series without getting frustrated or even angry. Of course I still know that there are books in the series that I didn't like at all and that the series may even go on, but I'm ok with it (I'm a master of repression :) ).
Some might think it's a bit crazy to just replace the ideas of the author with my own, but for me it works perfectly. Because this way I can still enjoy thinking about the books in the series I liked and might even reread them.
Do you sometimes wish that a series would have ended with an earlier book than it actually did? Do you sometimes create your own, "better" ending for a series in your mind?