Though the notes by Collingwood aren't aimed directly at the unitarity question for groups of real rank one, his survey 9.1 toward the end fills in some of the ideas with a lot of references to the literature up to then. In this treatment your group occurs in the framework of its 2-fold universal covering $\mathrm{Spin}(n,1)$. One 1974 paper The unitary representations of the generalized Lorentz groups by Ernest Thieleker is cited by Collingwood but is not included in the references of Baldoni-Silva and Barbasch, which lists instead a slightly earlier one. The later paper in Trans. Amer. Math. Soc. 199, 327-367, is freely available online at www.ams.org/journals/ and might (or might not) help to fill in more details.
What you get from any of the sources may be slanted more toward pure mathematics or towards physics, but my understanding is that the details about your special case haven't been essentially improved on since Collingwood's monograph appeared. In any case, that's likely to be the only extended exposition focusing on the real rank one case. The unitary dual is unavoidably complicated to arrive at, even in your special example, so the general theory in the background gets heavy at times.