What's the canon status of Marvel novels?
Here is a list of the novels that have been confirmed as canon (mostly through the official handbooks and indices):
Amazing Spider-Man: Mayhem in Manhattan (Amazing Spider-Man #186)
Avengers: The Man Who Stole Tomorrow (Kang handbook entry)
Avengers and the Thunderbolts (Avengers Index, Captain America Index)
Avengers Battle the Earth Wrecker (Earth's Solar System appendix, Blockbusters of the Marvel Universe appendix)
Daredevil: The Cutting Edge (Sister Maggie handbook entry)
Fantastic Four: Countdown to Chaos (Red Ghost handbook entry)
Fantastic Four: Redemption of the Silver Surfer (Tyannanus handbook entry)
Fantastic Four: War Zone (Negative Zone handbook entry)
Fantastic Four: What Lies Between (Psycho-Man handbook entry)
Generation X: Crossroads (Razorback, Trans-Sabal handbook entries)
Hulk and Spider-Man: Murdermoon (Hulk online handbook bibliography)
Incredible Hulk: Cry of the Beast (Hulk online handbook bibliography)
Incredible Hulk: Stalker from the Stars (Gamma Mutates online handbook bibliography, Hulk online handbook bibliography)
Spider-Man: Down These Mean Streets (Blackie Gaxton handbook entry, Amazing Spider-Man Index)
Spider-Man: Drowned in Thunder (Amazing Spider-Man Index)
Spider-Man: Gathering of the Sinister Six (Razorback, Sinister Six, Stilt-Man, White Rabbit handbook entries)
Spider-Man: Revenge of the Sinister Six (Rocket Racer, Sinister Six handbook entries)
Spider-Man: Secret of the Sinister Six (Big Wheel, Mr. Bumpo, Finisher, Hypno-Hustler, Sinister Six handbook entries, Amazing Spider-Man Index)
Spider-Man: The Darkest Hours (Morlun handbook entry, Amazing Spider-Man Index)
X-Men: The Chaos Engine (Corps, Cosmic Cube, Merlyn, Saturnyne, Sat-Yr9 handbook entries, Blockbusters of the Marvel Universe appendix, Uncanny X-Men Index)
X-Men: Shadows of the Past (Quist handbook entry)
X-Men: Soul Killer (Belasco, N'Garai handbook entries, Blockbusters of the Marvel Universe appendix)
X-Men & Spider-Man: Time's Arrow (Black Knight (Sir Percy), Growing Man, Immortus, Iron Man (2020 AD), Kang, Merlyn, Morgan Le Fay, Terminatrix handbook entries)
also the following short stories from anthology collections:
Five Decades of the X-Men: "Up the Hill Backwards" (Pete Wisdom handbook entry)
Ultimate Hulk: "A Quiet, Normal Life" (Nebulon handbook entry)
Ultimate Silver Surfer: "Broken Land" (Blockbusters of the Marvel Universe appendix)
Ultimate Silver Surfer: "What's Yer Poison?" (Mephisto handbook entry)
Ultimate Super-Villains: "Child's Play" (Mephisto handbook entry)
Ultimate Super-Villains: "Private Exhibition" (Painter of 1000 Perils handbook entry)
Ultimate Super-Villains: "To the Victor" (Kang handbook entry)
Untold Tales of Spider-Man: "Poison in the Soul" (Shocker handbook entry)
X-Men Legends: "Diary of a False Man" (Uncanny X-Men Index)
The novels published in the 1990s by Berkley shared an internal continuity via appearances by characters created especially for that series. Therefore one could argue that ALL of them are canon, even though only some of them are listed above.
For a complete listing of Marvel novels (at least through 2009) see here:
http://www.marvunapp.com/ohotmu/appendixes/novapp.htm