What is the nature of interstitial hydrides?
Nonstoichiometric compounds are simply compounds that cannot be represented with integer number of atoms. What the book probably is trying to say is that the hydrogen atoms do not simply fill the empty space between the atoms in the crystal. In other words, the hydrides will cause a change in the structure of the lattice. In some compounds, the atoms that are being inserted in the free space in the crystal will not cause any noticeable change in structure. The crystal may become slightly distorted. But it will retain its original shape before insertion of the new atoms. What the book is trying to say is that this not true for hydrides. Scientists used to think that this was the case, but it is not true.