I think you should take your own advice. Things change, jobs don't turn out to be what we expect. It is pretty old school to expect someone to take a job and stay there for their whole career. On the other hand, you don't want to appear to be flitting around aimlessly. But I think if you have good reasons to leave each job you have left, then that's all you need. I see no point in staying at a job just for the sake of staying there.

I'm in my third position since I finished library school. I was at the first job for 1.5 years, the second for 3 years, and I've been here for 2. My current job is in a great location (I have a walking commute!) and I will probably stick around, but I also have to think about career advancement. If an appealing department head position came up elsewhere, I wouldn't hesitate to apply. Opportunities don't appear every day and I have to think about my career, not just my job.

At least that is how I look at it. Those who are hiring for positions may look at it differently, but I would hope they can be sensible and open to hearing your reasons for the job changes.

I agree that you should apply.

There's another reason, though ... you may get the job (a very good thing!) and if you do not you're no worse off than you are now. Not applying means that you're giving away some probability of getting the job, what economists call an "opportunity cost."

Thanks to everyone for their input. You are saying a lot of things that make sense--and it does seem absurd not to apply for what really may be a dream job. And I do lose little by applying.

Should I address this issue at all in the cover letter?

I'm late to this thread, because I've been settling in to my third post-MLIS job in three years. I agree with those above who have said there is no good reason not to give it a shot. You never know until you get into a job if it is going to be anything like you hoped when you applied, and you have to take a long term view. I never really gave it too much formal thought, but my family and I did move across the country shortly after library school, with the plan that we would get a few years experience and hopefully move back. That's exactly what happened, but it was on the short end for the 3-5 years that I expected (and included two positions before returning).

I would not necessarily address this in your cover letter, but when I applied for my current job, I visited & had an informational interview because I was interviewing at another library nearby (and was living far away). After that meeting, I sent a thank you email & mentioned that I was impressed that the two people I had met had both been there over a decade, and while I have moved around a good bit in the early part of my library career, I was eager to make a long term commitment and this seemed like a place with a good working environment and where my family could be happy. So while I certainly understand your hesitation, there's no good reason not to give it a shot and see what happens. Just because you've tried a variety of positions, that just means you haven't found one that is a perfect fit yet. Even though I'm only 1 week into my new job, I definitely feel more comfortable about it than either of my last two jobs.

And by the way, I did hear from one colleague that my former director made a comment to him, something like, "he'll be there for a year then move on, he can't be happy anywhere." So don't be surprised if you do hear some backlash, second hand or not, but I just took it as further evidence that I made the right decision so I would no longer have to work with a person with such a negative attitude and, frankly, bad manners.

Good luck.