How is samsara nirvana? How is samsara nirvana? I have heard this mentioned in a couple books recently, but they never go into much detail.
From a Zen standpoint, the unification of samsara and nirvana cannot be comprehended by the discriminating mind.  Don't worry if you are having problems!  In fact, the more you try to rationalize it, the farther away you will be from truly comprehending it (to paraphrase Seng T'san's Hsin Hsin Ming).  

If you aren't a Zen student, this is going to sound a lot like the mumbo jumbo the school is famous for.  Apologies in advance!  All the same, putting samsara up against nirvana, and trying to see the distinction, is a product of the small mind.  Likewise, trying to intellectualize how they are the same is to miss what is meant by nirvana (and samsara too for that matter!).  Thinking will get you nowhere.  Only direct insight will show you the place where the boundaries between the two break down.  Emptiness must first be apprehended.  One then looks deeply into that emptiness.  Coming out of profound emptiness, and back to world of form, one directly perceives how the two are not distinct.  This is enlightenment.  And it takes years on the cushion to get there!  It also takes deep faith and trust.  Once that realization is reached, however, you know it just as well as the feeling of the shirt on your back.