Why doesn't he re-assume his old identity?
Henry Evans is wanted by the police for (erm...) crimes unknown. These crimes are evidently serious enough to merit the Alliance send nine police fliers after him and no less than 6 heavily armed men to arrest him at his home. We can reasonably assume that if he was to become Henry again, he would be immediately at risk of arrest and substantial sentencing.
Why doesn't he assume another identity?
Ignoring the fact that he may simply not want to (his childhood was awful) or that stealing someone's identity would be quite difficult (the last time he did it involved killing someone), the reality is that Derrial Book is sufficiently well known that he is recognised in public, a decorated military man with a long track record of service. If he were to assume a third identity, that in itself would be highly suspicious behaviour that could lead to his arrest and trial where his other identity (and possibly his crimes against the Alliance) would be uncovered.