Storm Front by Jim Butcher
Book One of The Dresden Files
Harry Dresden is the only Wizard with a yellow pages ad in the Chicago phone book. He hovers at the edge of financial survival trying to make a living as a magical private eye. Most of his clients don't believe in the magical part but he gets results. The stories are told first person in an updated noir style.
Lately he's been making the rent more months than not via consulting fees from the Chicago PD's Special Investigations Unit lead by Karrin Murphy. She doesn't altogether believe in magic either but if they find a body in the middle of a chalk circle or the like he can at least tell her what the murderer thought he was doing.
Unfortunately for Murphy magic is very real. There is an underworld (both figurative and literal) of Wizards, Vampires, Werewolves, Demons, Faeries, Ghosts, and assorted Things out there and in the last few years it's been getting more and more active. Even when it doesn't pay (often) Harry is driven to protect the innocents who get in their way.
Harry is aided by a being named Bob who is tied to a human skull that sits on a shelf in his workroom. Bob has a long memory and provides Harry with background information and potion recipes.
There was one short season of a Dresden Files TV series and it was after watching the DVD collection while housesitting that I put the book series on hold. I've been spacing them out at one per month ever since.
I was really surprised at how different the books are from the series. I expected some differences but not to the extent that there were. There are characters with the same names who serve the same functions who are completely different in appearance and personality. The TV show often combined elements from different cases into one and then gave it a different solution. Having experienced one or the other first will often not spoil the story at hand at all. In fact I think I was slower to pick up on some clues in this first book because it kept NOT going the way the similar situation was set up on TV.
I really liked this book but after having now gone through more of the books and gotten used to the changes I think I like the TV version better in a lot of ways. I still recommend the book series but I miss the relationship Bob and Dresden had on the show. Book!Bob is an air elemental who is forced to work for whoever owns the skull. He seems to have developed some real liking for Harry but the only thing keeping him loyal is a combination of the spells that bind him and his fear of what might happen to him without a Wizard's protection. He hints that he's made some nasty enemies through the centuries. He is a formless cloud of colored light during the brief times we see him transferring from the skull to possess Dresden's pet cat as a way to go out and investigate for Harry.
TV!Bob is the ghost of a human wizard who was sentenced to be bound to the skull by The White Council, the Wizard's governing body, after he tragically broke a major law trying to save someone he loved. He has a far bigger role than he does in books and often manifests as a ghostly middle-aged man. He has an old family retainer sort of relationship with Dresden. When Dresden was orphaned as a young child and brought to live with his uncle, Bob's previous keeper, Bob was given the job of Harry's tutor. There is real affection on both sides even though they frequently bicker and Bob is a master of snark.
Another character whose TV version I miss is Morgan, the White Council's Warden. In this book he's a somewhat unintelligent white man with greying hair. On TV he's a very intelligent young attractive black man. Book!Morgan is a burnt out cop who views everyone with a jaundiced eye. TV!Morgan is hard but fair. His lawful good view of the world makes dealing with someone with situational ethics hard and he's heard some nasty things (some true) about Harry that convince him Harry is biding his time before doing something punishable by execution.
Both versions come to have a grudging respect for Harry, enough to work together anyway, but there was a lot of potential for the TV version to become a real friend which is something Harry needs.
Between the books and the TV series Harry has different talismans, powers, allies, and enemies. They are so different that it is best to consider them unrelated things. I really wish there had been more episodes of the TV series made, especially since the book will not make up for them. At least there's always fanfiction.
If you're going to do both I'd recommend that you watch the TV series first. At least for me it's easier to deal with the subverted expectations of an AU version of something in print form than it is to change my reference points on the fly in the faster medium of film.