This can be skipped completely.
Despite the whole world thinking him dead, Magneto made himself known to the Avengers when he decided to come for his daughter the Scarlet Witch after her nervous breakdown, which led to her attacking the Avengers, leaving some members dead and Iron Man forced to disbanding the team. Magneto takes her to Xavier hoping he can help her. -summary
House of M Prelude: Excalibur written by legendary writer Chris Claremont takes place during the third volume of Excalibur series which ran for only 14 issues, with this book containing issues 11-14. For those who do not know, this volume happens to continue off of Grant Morrison's New X-Men run where the mutant island of Genosha was destroyed, with Xavier along with Magneto attempting to rebuild the island. Several wandering mutants make it to the island with former Morlock leader Callisto being among them. This book begins the dual storyline four parter The Devil's Own. One part of the story is a lackluster all out action tale that follows Madame Hydra aka Viper, as she attempts to kidnap the president of Zanzibar, which leads her into a battle with Archangel and Husk of the X-Men, as well as Callisto's small nameless group. The second story follows the events of Avengers: Disassembled, as Xavier has given up trying to help the Scarlet Witch and he now seeks aid from Dr. Strange.
The only really interesting portion of this story is the situation surrounding the Scarlet Witch. Xavier tries his best to help her, but soon learns from Strange that it's impossible to do so because he hasn't made peace with his own demons. An introspective story soon takes place as Xavier learns a few things about himself as well as Magneto. Magneto appears to have also given up, and he recognizes himself as a failure to both of his children, Scarlet Witch and Quicksilver. Claremont does display some of the clever and interesting writing that has made him legendary, by going into some type of depth explaining that the relationship between Magneto and Scarlet Witch goes further than father and daughter, that they are also linked by their extraordinary powers; Magneto possesses one of the fundamental forces of nature, while his daughter possesses complete control over reality. The story attempts to foreshadow events that would later take place in House of M, but you don't really learn anything here that isn't better covered later on.
Some of the problem here is that the House of M elements felt forced into this story only to help with the bad sales of this then Excalibur series. There's really nothing noteworthy here that actually plays into the main story. On top of this, the entire third volume of Excalibur can be considered a throw away series for two reasons; first of all, there is no connection in this volume with main Excalibur continuity, and second, the entire Genosha storyline is left hanging and eventually tied up much later in the X-Men books. I don't believe I'm wrong in saying that this entire Excalibur run can be completely skipped.
Aaron Lopresti's artwork is good, but it doesn't save the slightly average storytelling. There are some decent character designs providing the eye candy for some female characters such as Courtney Ross aka the White Queen. Viper has a decent design in her secret agent outfit, but it's tacky at best when compared to her skintight Hydra green. There is a fair amount of inconsistency with some characters having less details than originally seen. The action panels aren't what I would say to be passable; they felt pretty unimaginative and didn't hold my attention at all.
House of M Prelude: Excalibur is completely skip material unless you're a serious comic collector or Excalibur fan. Casual fans should stay far away from this unless they have vast knowledge of the X-Men universe. Many of these characters are C-list at best, and there's no real development here to grow a liking to them, that all took place in the first two volumes and it wasn't that great even there. If you want a prelude to House of M, then Avengers: Disassembled gives it all to you.
Pros:-Decent artwork and some story elements
Cons:Completely skippable for the most part