Well, this is a peculiar book, to be sure. 
 The novel starts off as a sort of coming-of-age tale about a young man, without any kind of real attachments, starting out in his first job when he begins noticing very strange things indeed happening to and around him. Now, because it is literally written in the book description, I knew that those strange things were actually happening in-universe, that this guy just got himself hired by a magical firm, but it took so long to be revealed in-book that for a while, I started to doubt myself and wondered if I was actually reading some kind of drama about a lonely, mentally-ill young man manifesting paranoid delusions and slowly descending into madness. 
 (I wasn't.) 
 So, a slow start. It then transitions into proper urban fantasy, with magic and wizards and fantastical creatures, and a supernatural artefact at the center of a mystery needing to be solved. All fun, all good things. 
 There is a but. 
 Some parts of the novel are hilarious, laugh-out-loud hilarious, even. I saw Tom Holt's writing style being described as being similar to that of Douglas Adams, and to be sure, there is something very alike, a particular kind of dry british wit that makes The Portable Door wickedly funny at times. It also deals with some very dark themes. For the most part, those elements come together pretty succesfully to deliver a cleverly written dark comedy. 
 Except for the whole part dealing with implied rape, that is. Specifically, (view spoiler)[there is a pretty big subplot about a "love philtre", i.e. a date rape drug by any other name, and not only is the protagonist sorely tempted to use it, trying to justify it to himself, but another character ACTUALLY uses it, which is painted as actually ok and even kind of wonderful. Direct quote: "What's there to be sorry about?" Paul said. "That's wonderful." In a word: ew. (hide spoiler)] That part was just baffling and somewhat infuriating, even. 
 It would have been a fun read if that last part hadn't soured the whole thing for me and left me with mixed feelings about it all. 
 In conclusion: -\_(tsu)_/-