I really would like to learn about the foundations of high-dimensional topology, the study of manifolds of dimensions 5 or greater. I learned the algebraic topology, but much of focus in what I learned in topology, such as geometric topology, has been low-dimensional manifolds. I concluded that I am intensely more interested about high-dimensional manifolds than low-dimensional manifolds; unfortunately, it seems that there are no introductory textbooks or monographs about high-dimensional manifolds.
Where should I begin with? What topics should I study that are core of high-dimensional topology? I have a feeling that surgery and cobordism theories are important, but I am not sure. Should I instead trying to read past research articles than specific textbooks?