I have finished most of the Horus heresy series, and thinking back, I wish I knew then what I know now in what order to read it. I personally read it in release order and I find it pretty sloppy and all over the place. While it is generally in chronological order, I didn't like how character arcs are interrupted with other stories and how long it takes for the story to return to said characters, because my memory is so bad. So with my proposed reading order, I decided to focus on characters and events. the highlighted names are the main books, while the unhighlighted names are tie-in short stories. In the end, this proposed reading order is by no means perfect and still probably is an unfinished mess and has much to improve and add, so don't be afraid to make suggestions. Enjoy!

1. Horus Rising - Dan Abnett
2. False Gods - Graham McNeill
3. Galaxy in Flames - Ben Counter
4. Fulgrim - Graham McNeill
5. Legion - Dan Abnett
6. Tales of Heresy: Wolf at the Door - Mike Lee
7. Tales of Heresy: Blood Games - Dan Abnett
8. War Without End: Howl of the Hearthworld - Aaron Dembski-Bowden
9. Prospero Burns - Dan Abnett
10. The Burden of Loyalty: The Thirteenth Wolf - Gav Thorpe
11. The First Heretic - Aaron Dembski-Bowden
12. Eye of Terra: Aurelian - Aaron Dembski-Bowden
13. Battle for the Abyss - Ben Counter
14. Know No Fear - Dan Abnett
15. Mark of Calth
16. The Burden of Loyalty: Perpetual - Dan Abnett
17. Legacies of Betrayal: Honour to the Dead - Gav Thorpe
18. The Honoured - Rob Sanders
19. The Unburdened - David Annandale
20. Legacies of Betrayal: Censure - Nick Kyme
21. Heralds of the Siege: The Ember Wolves - Rob Sanders
22. Tales of Heresy: After Desh'ea - Matthew Farrer
23. War Without End: Lord of the Red Sands - Aaron Dembski-Bowden
24. Legacies of Betrayal: Butcher's Nails - Aaron Dembski-Bowden
25. Betrayer - Aaron Dembski-Bowden
26. Legacies of Betrayal: Khârn: The Eightfold Path - Anthony Reynolds
27. Legacies of Betrayal: Heart of the Conqueror - Aaron Dembski-Bowden
28. Heralds of the Siege: Children of Sicarus - Anthony Reynolds
29. Age of Darkness: Rules of Engagement - Graham McNeill
30. Shadows of Treachery: Raven's Flight - Gav Thorpe
31. Age of Darkness: The Face of Treachery - Gav Thorpe
32. Deliverance Lost - Gav Thorpe
33. Heralds of the Siege: The Grey Raven - Gav Thorpe
34. Age of Darkness: The Iron Within - Rob Sanders
35. Eye of Terra: Ironfire - Rob Sanders
36. The Primarchs: The Reflection Crack'd - Graham McNeill
37. Legacies of Betrayal: Kryptos - Graham McNeill
38. Angel Exterminatus - Graham McNeill
39. Shattered Legions: The Seventh Serpent - Graham McNeill
40. The Outcast Dead - Graham McNeill
41. The Silent War: Wolf Hunt - Graham McNeill
42. A Thousand Sons - Graham McNeill
43. Legacies of Betrayal: Thief of Revelations - Graham McNeill
44. Legacies of Betrayal: Lucius: The Eternal Blademaster - Graham McNeill
45. The Crimson King - Graham McNeill
46. The Flight of the Eisenstein - James Swallow
47. Shadows of Treachery: The Crimson Fist - John French
48. Nemesis - James Swallow
49. War Without End: Gunsight - James Swallow
50. Garro - James Swallow
51. Tales of Heresy: Scions of the Storm - Anthony Reynolds
52. The Silent War: The Purge - Anthony Reynolds
53. Tales of Heresy: The Voice - James Swallow
54. The Silent War: Ghosts Speak Not - James Swallow
55. The Silent War: Patience - James Swallow
56. Fear to Tread - James Swallow
57. The Primarchs: The Serpent Beneath - Rob Sanders
58. Legacies of Betrayal: Brotherhood of the Storm - Chris Wraight
59. Scars - Chris Wraight
60. Eye of Terra: Brotherhood of the Moon - Chris Wraight
61. Legacies of Betrayal: Wolf's Claw - Chris Wraight
62. The Burden of Loyalty: Wolf King - Chris Wraight
63. War Without End: Daemonology - Chris Wraight
64. War Without End: The Devine Adoratrice - Graham McNeill
65. Age of Darkness: Little Horus - Dan Abnett
66. Vengeful Spirit - Graham McNeill
67. War Without End: Wolf Mother - Graham McNeill
68. War Without End: Imperfect - Nick Kyme
69. War Without End: Chirurgeon - Nick Kyme
70. Legacies of Betrayal: Warmaster - John French
71. Age of Darkness: Rebirth - Chris Wraight
72. War Without End: Allegiance - Chris Wraight
73. The Path of Heaven - Chris Wraight
74. Heralds of the Siege: The Last Son of Prospero - Chris Wraight
75. War Without End: All That Remains - James Swallow
76. War Without End: Black Oculus - John French
77. Tallarn - John French
78. Eye of Terra: The Eagle's Talon - John French
79. Eye of Terra: Iron Corpses - John French
80. Heralds of the Siege: Dark Compliance - John French
81. The Buried Dagger - James Swallow (to me this book fits here and also at the end, just after Titendeath)
82. Born of Flame: Promethean Sun - Nick Kyme
83. Born of Flame: Scorched Earth - Nick Kyme
84. Vulkan lives - Nick Kyme
85. Descent of Angels - Mitchel Scanlon
86. Fallen Angels - Mike Lee
87. Age of Darkness: Savage Weapons - Aaron Dembski-Bowden
88. The Primarchs: The Lion - Gav Thorpe
89. Shadows of Treachery: Prince of Crows - Aaron Dembski-Bowden
90. The Unremembered Empire - Dan Abnett
91. War Without End: The Laurel of Defiance - Guy Haley
92. War Without End: Virtues of the Sons - Andy Smillie
93. Eye of Terra: Sins of the Father - Andy Smillie
94. Eye of Terra: Herald of Sanguinius - Andy Smillie
95. War Without End: A Safe and Shadowed Place - Guy Haley
96. Eye of Terra: The Long Night - Aaron Dembski-Bowden
97. The Burden of Loyalty: The Heart of the Pharos - L. J. Goulding
98. Pharos - Guy Haley
99. Heralds of the Siege: The Painted Count - Guy Haley
100. Legacies of Betrayal: Cypher: Guardian of Order - Gav Thorpe
101. Eye of Terra: Master of the First - Gav Thorpe
102. War Without End: By the Lion's Command - Gav Thorpe
103. Angels of Caliban - Gav Thorpe
104. Heralds of the Siege: Exocytosis - James Swallow
105. Eye of Terra: Red-Marked - Nick Kyme
106. Eye of Terra: Stratagem - Nick Kyme
107. Deathfire - Nick Kyme
108. Shadows of Treachery: The Kaban Project - Graham McNeill
109. Mechanicum - Graham McNeill
110. The Burden of Loyalty: Cybernetica - Rob Sanders
111. Heralds of the Siege: Myriad - Rob Sanders
112. Eye of Terra: Vorax - Matthew Farrer
113. The Burden of Loyalty: The Binary Succession - David Annandale
114. Tales of Heresy: The Last Church - Graham McNeill
115. The Silent War: The Sigillite - Chris Wraight
116. The Burden of Loyalty: Into Exile - Aaron Dembski-Bowden
117. Master of Mankind - Aaron Dembski-Bowden
118. Heralds of the Siege: Magisterium - Chris Wraight
119. The Burden of Loyalty: Ordo Sinister - John French
120. The Primarchs: Feat of Iron - Nick Kyme
121. Shattered Legions: Meduson - Dan Abnett
122. Legacies of Betrayal: Riven - John French
123. Born of Flame: Artefacts - Nick Kyme
124. Born of Flame: Sons of the Forge - Nick Kyme
125. Old Earth - Nick Kyme
126. Shadows of Treachery: The Dark King - Graham McNeill
127. Shadows of Treachery: The Lightning Tower - Dan Abnett
128. Praetorian of Dorn - John French
129. Legacies of Betrayal: Veritas Ferrum - David Annandale
130. The Damnation of Pythos - David Annandale
131. Ruinstorm - David Annandale
132. Wolfsbane - Guy Haley
133. Heralds of the Siege: Duty Waits - Guy Haley
134. Heralds of the Siege: The Soul, Severed - Chris Wraight
135. Slaves to Darkness - John French
136. Corax: Weregeld - Gav Thorpe
137. Legacies of Betrayal: The Divine Word - Gav Thorpe
138. Heralds of the Siege: Valerius - Gav Thorpe
139. Titendeath - Guy Haley
140. Heralds of the Siege: Now Peals Midnight - John French
141. Heralds of the Siege: The Board is Set - Nick Kyme