The editors of each book bring together a large number of professional and influential philosophers, all contributing essays on specific topics within the aim of the book.
Usually, these books run for a little over a hundred dollars. But thanks to Amazon, and a bit of luck as well, I've been able to acquire the Handbooks that I do have for ~$40 or less.
So far, I have
I hope to get a few more in the future (although I need to finish reading the ones I do have first...), specifically the one on Philosophical Methodology, Ethical Theory, Bioethics, Death, Practical Ethics, and Atheism. No doubt this will take years to accomplish, even though I'm tempted just to spend what little extra money I do have on these books.
I definitely recommend this series of books, especially if you're interested in a general, yet in-depth, approach to a specific field.