Best way to get better at interviewing, is to do more of it, preferably with someone to critique it. I doubt that books are going to help much. It's a very on-the-job-training thing.
The more you interview, the more you get clarity about what you yourself value, and how you'd look for that in the candidate. That's true of coding as well as non-coding skills.
You are already getting a chance to interview several candidates at your job. I'd suggest you shadow some good interviewers for a few interviews. Pick someone who you think conducts an interview very well, and follow them in the interviews as well as debrief sessions. Stay quiet all through the interview and the debrief and take notes. After the debrief, talk with the person to reconcile any observations. That way you're not interrupting, you get ample thinking time of your own, and you also get to calibrate it with the person you were following. It's bit of an art.
Alternatively, if you can come to Sunnyvale, feel free to visit us: http://InterviewKickstart.com. We are an interview training bootcamp and do ~75+ interviews a month, all for core software engineering roles. I'm happy to have you shadow some with our very experienced interviewers.
(Me: soham@interviewkickstart.com)
