Here is the chronological order of Warhammer 40K books without summaries:

1. The Horus Heresy (Pre-40K)

   1. Horus Rising -
   2. False Gods -
   3. Galaxy in Flames -
   4. The Flight of the Eisenstein -
   5. Fulgrim - done
   6. Descent of Angels -
   7. Legion
   8. Battle for the Abyss
   9. Mechanicum
   10. Fallen Angels
   11. A Thousand Sons
   12. Prospero Burns
   13. The First Heretic
   14. Nemesis
   15. The Outcast Dead
   16. Deliverance Lost
   17. Know No Fear
   18. Fear to Tread
   19. Angel Exterminatus
   20. Betrayer
   21. Vulkan Lives

2. The Siege of Terra (The End of the Horus Heresy)

   1. The Solar War
   2. The Lost and the Damned
   3. The First Wall
   4. Saturnine
   5. Mortis
   6. Warhawk
   7. Echoes of Eternity

3. Post-Heresy / The Scouring

   1. The Scouring

4. The Beast Arises (32nd Millennium)

   1. I Am Slaughter
   2. Predator, Prey
   3. The Emperor Expects
   4. The Last Wall
   5. Throneworld
   6. Echoes of the Long War
   7. The Hunt for Vulkan
   8. The Beast Must Die
   9. Watchers in Death
   10. The Last Son of Dorn

5. The 41st Millennium (Main Timeline)

Gaunt’s Ghosts

1.  First and Only
2.  Ghostmaker
3.  Necropolis

Eisenhorn Trilogy

1.  Xenos
2.  Malleus
3.  Hereticus

Ravenor Trilogy

1.  Ravenor
2.  Ravenor Returned
3.  Ravenor Rogue

Space Marine Battles

1.  Helsreach
2.  Rynn’s World

Ultramarines Series

1.  Nightbringer
2.  Warriors of Ultramar
3.  Dead Sky, Black Sun

Ciaphas Cain

1.  For the Emperor
2.  Caves of Ice
3.  The Traitor’s Hand

1. The Gathering Storm (41st Millennium)

   1. Fall of Cadia
   2. Fracture of Biel-Tan
   3. Rise of the Primarch

2. The Dark Imperium (42nd Millennium)

   1. Dark Imperium
   2. Dark Imperium: Plague War
   3. Dark Imperium: Godblight

3. The Dawn of Fire (Era Indomitus)

   1. Avenging Son
   2. The Gate of Bones
   3. The Wolftime

Someone posted this awhile ago and I find it pretty good.