A reference request for zero-order hold (ZOH) From Wikipedia:


  A zero-order hold reconstructs the following continuous-time waveform
  from a sample sequence $x[n]$, assuming one sample per time interval
  $T$: $$x_{\mathrm{ZOH}}(t)\,= \sum_{n=-\infty}^{\infty} x[n]\cdot
 \mathrm{rect} \left(\frac{t-T/2 -nT}{T} \right) \ $$ where
  $\mathrm{rect}()$  is the rectangular function.


I'd like to find a good bibliographic reference for it.

Thanks in advance.
I'd personally start with "Signals and Systems" by Oppenheim and Willsky. I have the second edition, and it has a good description of zero-order hold. This book is a must-have if you're interested in signals and systems. There is also a version that focuses on discrete signals. In addition, this book is probably available at most engineering libraries.