An instance where "was" sounds natural:
"Although burglars succeeded in penetrating the security, my book was left untouched."
An instance where "is" sounds natural:
"The exam was postponed, my book is, again, left untouched."
The difference between these two might be that in the second instance, the news of the exam being postponed have just been received. Quite possibly the book is within reach.
In the first instance relatively more time has passed since the burglary. It is less recent and the book is probably not within reach.
I'm trying to justify the reason these two things feel natural to me in different situations. I hope it helps.