You can't go wrong with Rabaey's book... I found it on Amazon, you can take a look:

http://www.amazon.com/Design-Essentials-Integrated-Circuits-Systems/dp/0387717129/ref=sr_1_1?ie=UTF8&qid=1396051326&sr=8-1&keywords=rabaey+low+power

Thank you Saha.

u could refer low power cmos vlsi design, by kaushik roy

Thx J.D. Warnock

I think it is one of the best books

Although Prof. Rabay's book is indeed an excellent book, it mainly discusses circuit-level issues and involve transistors and VLSI. However, I believe that the current trend is to exploit system level techniques, e.g. DVFS, and DPM, finding energy bugs, etc.. When you use system level techniques, you do not involve transistors and VLSI (or you may need just a brief overview), rather you involve scheduling problems, algorithms, and optimization problems.