I would like to focus on programming for mobile devices such as iPhone, Palm Pre and last Nokia series. I'm interested in usability, user interface design, memory management, application design... every single part of it.
What books do you recommend for this topic? What are the must have books for mobile device programming?
Unfortunately due to varied platforms (Android, iPhone, J2ME, etc.), it's hard to find a "one-stop-shop" book on mobile development. From what I've seen in the current market, you're almost forced to choose your initial platform and then hammer away.
Depending on your background, you may have a smaller learning curve with different platforms. If you have more high-level language experience (Java, C#, Python), ramping up on Android is relatively painless.
From personal experience, I've found iPhone development to be the biggest pain, mainly due to the ginormous iPhone SDK and having to learn Obj-C solely to code for OS X.
To help narrow it down, I've found Pragmatic Programmers to be a solid publisher. Their iPhone and Android titles get you up to speed quickly without having to read 500+ pages of API docs. Manning is pretty good too, but slightly more verbose than Pragmatic. Either way, they both put out solid material. Apress also has some decent titles to take you from 0-60 in no time.
Again, pick your poison and dive in :)
You mention usability, so I guess you're interested in broader issues of usage as well as pure programming issues. On that basis, I'd recommend a look at Adam Greenfield's Everyware, which is a collection of short essays on how people are using ubiquitous computing and how it's likely to affect our interactions over the next few years.
It won't give you any programming tips but it's a thoughtful book about the field in general. Greenfield takes a high level look at some of the implicaations of ubiquitous computing and it might help spark ideas for application development or put some aspects of mobile interaction into a broader context.