Reddit mentions of Halo: Primordium: Book Two of the Forerunner Saga

Nope. Read the second book in the Forerunner series.

As far as I know the chronological order is:

• Halo: Cryptum
  
  
• Halo: Primordium
  
  
• Halo: Silentium
  
  
• Halo: Contact Harvest
  
  
• Halo Wars
  
  
• Halo: The Cole Protocol
  
  
• Halo: Reach
  
  
• Halo: The Fall of Reach
  
  
• Halo: Combat Evolved (Anniversary)
  
  
• Halo: The Flood
  
  
• Halo: First Strike
  
  
• Halo: Ghosts of Onyx
  
  
• Halo 2
  
  
• Halo 3: ODST (actually in the middle of Halo 2 if you want to follow it perfectly)
  
  
• Halo 3
  
  
• Halo: Glasslands
  
  
• Halo: The Thursday War
  
  
• Halo 4
  
  But then several books occur over long periods of time, so it makes them hard to place. If you've played Halo: CE recently you might want to skip The Flood, as it's basically the same plot with some extra stuff along the way.
  
  You can read them in that order or in release order, as someone else said. Maybe release order the first time, and chronological on the second read? When I read them a while ago I think I ended up with something of a mix, but either way they are an amazing read.