I'm finding Cicero's De Officiis a very helpful workout, especially in two areas.
The first is pronoun practice. "de officiis" translates "on duties". Since duties are always duties to somebody, or something, this provides the pronoun practice. Pronouns give me lots of problems, so I definitely need the practice.
The second is in getting used to the periphrastic constructions. Since duties are moral obligations, there are many of these constructions in "de officiis".
As compared to poetry, the word order presents fewer difficulties for one whose mother tongue is English.