As an addict of 'Magic the Gathering' novels in my youth and a dabbler of Blizzard Entertainment book tie-ins, I've grown up with a bit of a prejudice against books adapted from other products. They never really seem to capture the same feeling you had while playing the video-game/board game/watching the show/etc, and when they're not being out-right terrible and distorting the entire canon as you thought you had it figured out from snippets of flavor text, they tend to hit you over the head with unnecessary references.
The Arkham Files book tie-ins are not really good books, but here I am with four of them in my house now and another ordered and on the way, so they must be doing something right.
If I had to guess, I'd say that 'something' is the very particular genre-blend that Arkham Horror accomplishes of putting 1920's jazz and prohibition, gangsters and feds, guys and dolls aesthetic fun together with lovecraft's big bag of monsters, and stirring in a healthy ladle-full of Robert E Howard's heroics. Lovecraft purists who whinge about a lack of nihilistic, existential horror will certainly not be made happy, and If the big man himself was brought back to life and shown what Fantasy Flight Games was doing to his work I think he'd probably lock himself up in his apartment and spend a good week writing a scathing letter or two about 'decadence' and 'cultural decline'. But Lovecraft also thought that Africans, Jews and just about everyone but white academics were par with inbred fish-people, so who cares what he would have thought? I like my Arkham Files junk food, and if it keeps me busy while I wait for the next LCG cycle to come out, then I'll happily keep on buying these books.