The relevant chapters of Diodorus Siculus' Library of History are perhaps the most complete account we have of events from 323-301. Diodorus was a Sicilian Greek historian of the first century BC whose work is essentially a large compendium of world history. He isn't perfect and is much critiqued but I've always thought him to be perfectly serviceable. His work is based on the writings of Hieronymus of Cardia, a friend and possible relative of Eumenes, who lived through many of the most tumultuous events of this stormy period.

Although he was a much later writer Plutarch can also be helpful here. His Lives of Eumenes and Demetrius both give interesting accounts of these two vital figures. See also the Life of Pyrrhus. However, Plutarch did not consider himself a writer of serious history but more of a moralizing biographer. Keep this in mind when reading him.

You can also look to Justin but be careful. This late Roman writer is notorious for his sloppy work. Use him to supplement your knowledge and fill in some gaps but not as your go-to source.

For accounts of battles and military affairs specifically you can also try Polyaenus. This obscure 2nd century Greek writer wrote a considerable amount on warfare and his descriptions of key engagements seem grounded in fact. I'm not very familiar with him though so I'll say no more.

Lost works have been summarized for us by the ninth century Byzantine cleric Photius. These include summaries of accounts by Arrian and Memnon of Heraclea. The latter is focused, unsurprisingly, on Memnon's hometown which played an important role in later Diadochi conflicts.

Finally, for some interesting tidbits on the career of Seleucus you can look to the relevant sections of Appian's Syrian Wars. It's a bit brief and dotted with fancy but still worth looking at.