I have read for example in the introduction of http://arxiv.org/pdf/math/0506338v2.pdf about the property that if we glue hyperbolic manifolds with geodesic boundary consisting of tori along some tori, the volume of the manifold we obtain is simply the sum of the volumes of the pieces.
The only reference i found for this result is Thurston's book Geometry and topology of 3-manifolds.
However, Proposition 6.5.2 of the book seems to prove only an inequality. I think the other inequality might be a consequence of Theorem 6.5.5 in the same book but the proof of this result is missing.
Therefore i would like to find a precise reference for this property of additivity of the simplicial volume.