Evil Dead/WMG

The whole story is one big stable time loop.
Yeah, I know, that one's pretty obvious, but hear me out: Ash was demonically possessed TWICE and got better, and also had his hand possessed, and all his loved ones were killed by it. Further more, In Army of Darkness, the eponymous army is lead by an Evil version of him that seems to share alot of his personality traits. Oh, and he's apparently "the man from the sky" destined to fight Deadites. Thus, the only conclusion I can come up with is this: the Deadites KNEW he was the one who stopped them, so they tried to kill him (or at the very least, drive him insane). In short, the Deadites pretty much screwed themselves over by driving Ash to become the Badass he ended up becoming.
 * I'd say that's canonically true, especially in the second movie. Ash felt a chill and said it felt like "someone walked over [his] grave" when Annie showed him the picture of the Hero From the Sky, and the Deadites at the end seem to be gloating a little too much to have just beaten an average guy: "we've won, we've won! Victory is ours!" I think they quickly recognized who he was (since they've faced him before, in the 12th century), and thought at the end that they'd broken the time-loop and stopped him from ever sealing them away. But Because Destiny Says So, Annie manages to open the portal with her dying breath, and we all know what happened next...
 * That would explain why the necronomicon was found in the Brittish castle ( it was the same one from army of darkness), so would that mean that the book isn't summerian the an uncreated eternal of time travel? or is this an unstable timeloop that will play out differently evry time? or is there now a time travel duplicate book?

The Ash you see lives in a different continuity in every movie
In the first, Ash is a college student traveling with his friends to a creepy cabin in the woods, and at the end burns the Necronomicon/Demonicon, which is alive and well throughout the second movie. In the second, Ash only brings his girlfriend and perseveres through different -- but also similar -- events in the recap at the beginning. His shotgun also goes from a single-shot to a double-barrel and instead of an axe he acquires a full-out chainsaw. In the third, Ash undertakes a massive personality shift and the events he undergoes at the beginning are different than those at the end of the second movie. Also to be taken into account is the game Evil Dead: Regeneration, which is set in another alternate universe, where Ash is committed to an asylum instead of going back to the Middle Ages. All of this means either NOTHING is canon in the Evil Dead movies, or EVERYTHING is.
 * I'm gonna go with "Everything is", considering how much he's crossed over with in the comics. Not to mention all of reality has been taken over by Evil Ash, and since rewritten, twice now. (As of Issue 27)

In the first movie, the camera-monster is entirely unrelated to the Book of the Dead
The book possesses people with spirits and makes the trees come alive. The camera-monster is seen before anyone even touches the book and kills Ash after the book is burned. The events just happened to take place in a location with a camera-monster and a possessed tome.
 * Taking only the first film as "canon" in this case: Perhaps the book was something that bound the camera monster to it, yet somehow (for some reason) kept it from harming anything. That brings up the question of how it toppled trees when the book was still whole, but also explains why the professor didn't burn the thing.

The Lamia is a Deadite.
When it possesses people, their face morphs and they have the ability to levitate, just like a Deadite.
 * The Lamia also torments its victim and takes pleasure in doing so - much like a Deadite, as well.

Ash is actually telling the story of the entire trilogy to his coworkers
The differences in continuity are because by this point Ash is, shall we say, slightly less than mentally stable?

Ash never recovered from his first Deadite possession.
Ash IS a Deadite, now, or rather, the specific deadite and him... came to an agreement. Ash just happened to get possessed by the one who found out killing its own kind was FAR more entertaining.

There is no continuity error in the movies or the games, Ash is just crazy.
Each movie and game after the first Evil Dead tells of a diluted version of his origin. While the first movie shows him with his friends, the second and third only show him and Linda during flashbacks. The first and second movies shows them as College graduates, the third shows them as former (Ash being current.) S-Mart employees. Granted one can argue that they have the jobs to pay off their student loans, but it wasn't mentioned until Army of Darkness. The famous laughing scene has been diluted in more ways than one: First of all it was heavily implied that the stress from his fighting the deadites has made him insane, and subject to hallucinations. While in the game Evil Dead: Regeneration has shown it was the deadites and ash had to shoot them all. Also in the game unlike the scene in the movie, he is seen with his chainsaw hand on, while in the movie; moments away from Knowby's daughter (whom helped him build the chainsaw.) from arriving, he only had his stump and the boomstick. In the game as well there have been no signs of the supporting characters from the second movie as either characters or deadites while there were plenty first movie deadites to kill. Lastly and this is the big one to prove my point. There was no narrator for Evil Dead, Knowby narrated Evil Dead 2. Who narrated Army of Darkness and so on? Ash Williams. Though it is hinted that the deadites and the killings are true, however despite being the hero, he is the one responsible for diluting his own story. Evil Dead 1 had hints of him going crazy with the mirror water. Evil Dead 2 had him being strangled by his reflection while inanimate objects laugh at him. Army of Darkness had an elaborate story involving time travel and a demon war. The games just piled on the crazy. There is no continuity error of the series. Ash has been so screwed up by the terror of the deadites that even he can't get his story straight. Now if a Bruce Campbell look-alike went up to you and said. "Hey man did I ever tell you the story about the magic book and the demons posessing my friends, and I killed them with a chainsaw. Then I traveled back to the past and King Arthur and I have been fighting demons?" Would you believe that story?

Ash is related to Rory.

 * That level of badassitude? Gotta be genetic.