Board Thread:General Discussion/@comment-2602:306:362A:8C70:D4B1:A8F7:8EA0:13F4-20170718190729/@comment-25942629-20170728230454

You can read the manga alone and be okay. However, the LNs provide the key basic information on the Hiragi, the Ichinose, Mahiru, Cursed Gear, the Apocalypse, Shinya's background and the background of Guren and his team members, the Hyakuya Sect, and vampires. This information tends to be really limited in the manga.

The LNs are a prequel, so it's not like the manga covers the same storyline or anything.

If you want to fully understand what is going on in the mana, I strongly recommend reading the LNs. If you like Seraph of the End but not quite that much, then you'd be okay without it.