The Casefiles series, aimed at older readers, was published by Archway Paperbacks (an imprint of Simon & Schuster) between 1987 and 1998.

1. Dead On Target 2. Evil, Inc. 3. Cult of Crime 4. The Lazarus Plot 5. Edge of Destruction 6. The Crowning Terror 7. Deathgame 8. See No Evil 9. The Genius Thieves 10. Hostages of Hate 11. Brother Against Brother 12. Perfect Getaway 13. The Borgia Dagger 14. Too Many Traitors 15. Blood Relations 16. Line of Fire 17. The Number File 18. A Killing in the Market 19. Nightmare in Angel City 20. Witness to Murder 21. Street Spies 22. Double Exposure 23. Disaster for Hire 24. Scene of the Crime 25. The Borderline Case 26. Trouble in the Pipeline 27. Nowhere to Run 28. Countdown to Terror 29. Thick as Thieves 30. The Deadliest Dare 31. Without a Trace 32. Blood Money 33. Collision Course 34. Final Cut 35. The Dead Season 36. Running on Empty 37. Danger Zone 38. Diplomatic Deceit 39. Flesh and Blood 40. Fright Wave 41. Highway Robbery 42. The Last Laugh 43. Strategic Moves 44. Castle Fear 45. In Self-Defense 46. Foul Play 47. Flight Into Danger 48. Rock 'n' Revenge 49. Dirty Deeds 50. Power Play 51. Choke Hold 52. Uncivil War 53. Web of Horror 54. Deep Trouble 55. Beyond the Law 56. Height of Danger 57. Terror On Track 58. Spiked! 59. Open Season 60. Deadfall 61. Grave Danger 62. Final Gambit 63. Cold Sweat 64. Endangered Species

65. No Mercy 66. The Phoenix Equation 67. Lethal Cargo 68. Rough Riding 69. Mayhem in Motion 70. Rigged for Revenge 71. Real Horror 72. Screamers 73. Bad Rap 74. Road Pirates 75. No Way Out 76. Tagged for Terror 77. Survival Run 78. The Pacific Conspiracy 79. Danger Unlimited 80. Dead of Night 81. Sheer Terror 82. Poisoned Paradise 83. Toxic Revenge 84. False Alarm 85. Winner Take All 86. Virtual Villainy 87. Dead Man in Deadwood 88. Inferno of Fear 89. Darkness Falls 90. Deadly Engagement 91. Hot Wheels 92. Sabotage at Sea 93. Mission: Mayhem 94. A Taste for Terror 95. Illegal Procedure 96. Against All Odds 97. Pure Evil 98. Murder by Magic 99. Frame-up 100. True Thriller 101. Peak Of Danger 102. Wrong Side Of The Law 103. Campaign Of Crime 104. Wild Wheels 105. Law Of The Jungle 106. Shock Jock 107. Fast Break 108. Blown Away 109. Moment Of Truth 110. Bad Chemistry 111. Competitive Edge 112. Cliff-Hanger 113. Sky High 114. Clean Sweep 115. Cave Trap 116. Acting Up 117. Blood Sport 118. The Last Leap 119. The Emperor's Shield 120. Survival of the Fittest 121. Absolute Zero 122. River Rats 123. High Wire Act 124. The Viking's Revenge 125. Stress Point 126. Fire In The Sky 127. Dead In The Water

In an attempt to attract older readers, the Casefiles featured a "new-and-improved" Frank and Joe who, as an advertisement claimed, "find themselves investigating international terrorism, espionage rings, religious cults, and crime families. Whether they're in Europe or Bayport, the Hardy Boys are never far from life-or-death action."

The Casefiles were a drastic change from the established Hardy Boys world. Some of their major features:

• Iola Morton is killed by a terrorist car bomb in the first volume.
• The books become more grisly. Frank and Joe use firearms and investigate murders.
• The glamourous aspects of international intrigue are played up.
• Frank and Joe take a more serious interest in girls.
• The boys embrace high-tech gadgetry, and Frank becomes a computer whiz.
• Meanwhile, Joe becomes a dim-witted roughneck.
• The writing is bad.
• Chapters no longer have titles, and there are no illustrations.
• The boys collaborate with the Grey Man, who represents a cloak-and-dagger crimefighting unit. (This is almost as cheesy as SKOOL and UGLI from The Secret Agent on Flight 101...)
• Finally (and most unforgivably), Joe swears. This is even worse than carrying guns. The Hardy Boys don't swear!

Iola's death seems to have served two functions. First, it gives Joe a chip on his shoulder, allowing him to become a brutish S.O.B. Second, it removes the impediment of a steady girlfriend, allowing him to become a jealous womanizer. This is not the cheerful, impetuous Joe we knew and loved in the original series!

Fun facts:

• Number of times the word "mystery" occurs in a Casefiles title: 0.
  Number of times it occurs in an original series title: 17.
• Number of times the words "dead" or "death" occur in a Casefiles title: 10.
  Number of times they occur in an original series title: 0.

I think that neatly sums up the differences between the Casefiles and the original Hardy Boys books.

Covers

Examples (click for full-size image)

#1, 1987

#36, 1990

#74, 1993

#99, 1995

#121, 1997

As you can see, flames figure prominently in many Casefiles covers!

The cover of volume 121 features Paul Popowich as Joe and Colin Grey as Frank, as they appeared in the short-lived TV series of 1995. All of the Casefiles covers from this period featured Popowich and Grey in various mysterious poses that had very little relevance to the events of the book.