I have a reasonable background in undergraduate real analysis, and I am very interested in questions concerning the convergence of series, and also how this relates to irrationality measure. I would like to know what the current state of modern research on convergence is, my end goal being to fully understand all of the research in this area. I have encountered a few papers online that contain very interesting results that establish the convergence of, or convergence tests for, a wider class of series (relaxations of the monotonicity requirement in the integral test, for example), and have seen a few things about how the convergence of series can relate to irrationality measure. Unfortunately, I have found that I generally do not have the prerequisites for reading these papers, and each one references so many others that I feel I don't have any grasp of what the overall research on the subject looks like. So, are there any good monographs or textbooks I can read to gain an understanding of modern research on the convergence of series?
To clarify, I already have an undergraduate-level education in real analysis, so I'm not looking for straight analysis textbooks, unless they have a heavy emphasis on convergence of series.