would be H.H. Scullard's From the Gracchi to Nero, then two collections of essays, Donald Kagan's Problems in Ancient History, Volume Two: The Roman World and John Riddle's Problems in European Civilization--Tiberius Gracchus: Destroyer or Reformer of the Republic?.

All are a bit old, with publication dates of 1959, 1966 and 1970, respectively, so you might have some trouble finding them. I got Kagan's book back in college and Riddle's just a bit ago on eBay, I think, and have not had time to read it. The essays in Kagan's book are by Hugh Last, Jerome Carcopino, Henry C. Boren and R.E. Smith. Riddle's includes some of these same but also, among others, Theodor Mommsen, Matthias Gelzer, E. Badian and Lilly Ross Taylor.