Are you sure this book was left there for purposes of proselytizing? That is, is it a tract? Cause if so it sounds like a yawner. A book on the Doctrines and Covenants, though, sounds like the type of material he would be reading for his own edification. What if you accidentally left a copy of The Orthodox Church or perhaps St Athanasius' On the Incarnation of Christ in the breakroom? Would you want someone to turn you in to HR, throw it out, or would you want them to return your book?
WWJD is all well and good for late '90s Evangelical fashion statements, but the real question is "How would I want to be treated in a similar circumstance?" You know, the Golden Rule.