Usability
Clinical decision support is equal parts aiding the clinician to 'automagically' make the right decision and presenting options in a way that is intuitive. Being afforded the luxury to design the user interface of clinical design support advisors also brings up unique challenges. The clinical information may be very precise and accurate, but there is a need to present it in a way that facilitates a good user experience.
Speaking to programmers and developers, there is one usability book that is universally recommended: "Don't Make Me Think" by Steve Krug.

I'm going to pick up a copy and review it shortly.
Whether you hate Apple or love them, they also have a very good set of usability guidelines here.
via rxinformatica

Comments
Ok, but there are better
This is a good book for web site usability. But there are better books if you don't want to jump to the assumption that a web based interface is the way to go. Donald A. Norman's "Design of Everyday Things" is where I started my interest in the topic and I'd recommend it if you haven't read it already.
@gauti
Thanks for the recommendation
Thanks for the recommendation gauti! We have a heavy web based interface for our EHR and CPOE so my recommendation was partially biased :)
More on Pharmacy Usability
Nice Post. I just got some material on a iTunes pharmacy profile from Allen Flynn. Will be posting soon. You are going to love it!