Mousey,
Understanding Linux Kernel 3rd edition explains the architecture of the kernel, its various parts, and for each, how it is designed and how it works.
OTOH, Linux kernel development is a book about programming the kernel, explaining various parts of the kernel and kernel API, how they work and how they should be used.
So their respective titles are actually accurate: The first one allow you to understand the kernel both as a whole in each of its main parts, while the second one is focused on kernel software development.
As far as I'm concerned, I have both, and I consider that I need both...
Now there are others:
Professional Linux Kernel Architecture (Wrox) is an excellent book, that if not as known as it should, IMO. It's very big and contains a lot of very useful, dense, deep inside information and explainations about various parts of the kernel internals. And it's quite recent, which is a serious point to consider. It helped me a lot in a number of occasions.
And if you are considering writing Linux Device Drivers, there are also of course
Linux Device Drivers 3rd edition (O'Reilly),
a great reference, even if it would benefit from an updated edition, and
Essential Device Drivers (Prentice Hall).
I realize that this adds to the complexity of your choices, but the fact is that those books are among the only serious, exhaustive technical pieces of documentation and manuals we have about Linux kernel programming...
I doubt that you'll be able to find Linux kernel development 3rd Ed as a used book, since it was just released three weeks ago. But if you need them, all the others might be available significantly cheaper as used books on Amazon, eBay and other places.
Good luck!