This study conducted a systematic literature review of 128 highly cited research papers on augmented search generation (RAG) published from 2020 to May 2025. Data were collected from databases including ACM Digital Library, IEEE Xplore, Scopus, ScienceDirect, and DBLP and analyzed according to the PRISMA 2020 framework. RAG combines neural network-based retrieval models with generative language models to leverage up-to-date information while preserving semantic generalizations stored in model weights. This study categorizes datasets, architectures, and evaluation methods, and synthesizes empirical evidence on the effectiveness and limitations of RAG to clarify the current state of research, highlight methodological gaps, and suggest directions for future research priorities. For papers published in 2025, we lowered the citation count threshold to include recent, groundbreaking research.