Reinhard Zimmermann's The Law of Obligations: Roman Foundations of the Civilian Tradition (Clarendon, 1990) seems to be exactly what you're looking for, since it not only gives a good overview of Roman law but is an inherently comparative work that traces its influence on modern legal systems. But it assumes some prior legal knowledge on the part of the reader, so a more introductory text may be more readable for someone without any legal background (e.g. du Plessis, Borkowski's Textbook on Roman Law (6th ed., OUP 2020)).

Zimmermann also discusses the evolution of Roman law from the Classical (early Empire) period to the Justinianic period, though it should be noted that Justinianic reforms centred around clarification and codification (e.g. picking a definitive stance on legal questions that had previously divided jurists based on which of the two main schools they belonged to) rather than innovation -- as such the difference between the substance of Roman law in, say, 150 CE (when Gaius was writing) and the time of Justinian's codification in 535 CE wasn't monumental.