In this post, I will review some textbooks on relativistic quantum mechanics (RQM). Most quantum field theory (QFT) books will briefly discuss some RQM in the beginning and why it is not a correct theory and will move on to QFT. Here I will only review those which discuss RQM somewhat completely. RQM is a one-particle theory. The reason it is hard to find books on RQM is because it is not a well defined theory like QFT.
Best introductory books

Best introductory books

1) Wachter, Armin (2011). Relativistic Quantum Mechanics. Theoretical and Mathematical Physics series. Springer.

This book is organised into 3 chapters with the 1st on spin 0 particles, the 2nd one on spin 1/2 particles and the 3rd one on scattering theory. It is organised very neatly.

2) Greiner, Walter (2000). Relativistic Quantum Mechanics. Wave Equations (3rd ed.). Springer.

Greiner has written many books on QM and QFTs. This book is a bridge between his QM and QFT books. Unlike the above book it discusses spins higher than 1/2 also. But for spin 0 and spin 1/2 it discusses slightly less completely than the above book.

There are several QFT books that also decently discuss RQM at the beginning like Lectures on Quantum Field Theory by Ashok Das (up to chapter 4), Gauge theories in particle physics by Aitchison and Hey (Vol 1 chapter 3), Quantum Field Theory by Lewis Ryder (chapter 2), Introduction to Quantum Field Theory Classical Mechanics to Gauge Field Theories by Anthony G. Williams (chapter 4) etc.