If you want to understand signals for research, read Oppenheim. If you want to pass an exam and rank in GATE, you need Rawat .
The book is structured into 12 primary chapters, progressing from basic definitions to advanced transformation domains: Introduction to Signals and Systems Signal And System By Tarun Kumar Rawat.pdf
While other books separate Laplace from Fourier, Rawat shows how Laplace generalizes Fourier for unstable systems. The Region of Convergence (ROC) charts in this PDF are particularly famous for their simplicity. If you want to understand signals for research,