Source of GMAT study material

Without any doubt, your most dependable source of practice material for GMAT is the Official GMAT Guide (latest edition is the 2019 edition). This book contains 900  past GMAT questions with detailed solutions. There is also some conceptual explanation at the start of the sections, but it is not very elaborate.

The Official GMAT guides are available as a set of 3 books. In addition to the book mentioned above, which covers both Quantitative and Verbal, there are 2 separate guides for Quantitative and Verbal respectively. Each guide contains 300 retired GMAT questions from that section.