You can get a excellent sense of Warhol's progress from this book. 1962 seems to have been a decisive years. Before 1962, one can see Warhol's transition from commercial artist and early experimentalism. Beginning sometime in 1962, one sees the emergence of the well-chosen, well-executed images that Warhol is known for.
It seems helpful in understanding his growth to see some of Warhol's less appealing works. Nonetheless, with a total of about 320 pages of images, there are still plenty of Warhol's bettter works to see here.
Four high-quality, significant essays about Warhol open this book. The closing includes a chronology, a "collective portrait" consisting of short contributions from many who knew Warhol well, and "Warhol in his own words", selections that reveal how insightful yet straight-forward Warhol could be.
This seems to be the single best bible of Warhol's paintings. There is a comprehensive collection of Warhol's prints available in "Andy Warhol Prints: A Catalogue Raisonne: 1962-1987" which seems prettier but may suffer from excessive prettiness. Warhol's trashier aspects are not apparent, nor is his experimental reach, in the prints. Both books have their appeal, but as a one source collection of Warhol's painting and critical assessments of it this Retrospective seems unparalleled.
For a good exposure to Warhol in all his diversity, "Andy Warhol: 365 Takes" by the staff of the Andy Warhol Museum is also valuable, but to focus on the paintings, this retrospective seems ideal.