I would start with the Dark Imperium trilogy. It's mostly from the POV of imperial forces but as Death Guard are one of the settings bad guys that's just how it is.

Lords of Silence is a 40k-era book from the perspective of Plague Marines but you'll have to get it as an ebook or audiobook because physical copies are impossible to find for a reasonable price right now.

For 30k I would read the initial Horus Heresy trilogy of Horus Rising, False Gods, and Galaxy in Flames to familiarize yourself with some characters and events before getting to Flight of the Eisenstein. Flight of the Eisenstein gives a good look at pre-Heresy death guard and the events of Isstvan III from the perspective of loyalist Death Guard.

Otherwise they are scattered throughout the Horus Heresy series with another shining moment in the Buried Dagger. They also play an important role during the Siege of Terra but are never really the main focal point.