Sometimes it's difficult to know exactly why a book was so enjoyable, and this is one of those books. I'll try my best to explain, but I heavily recommend reading this.
First, let's get the negatives out of the way. I almost rated this 4 stars, because there is some confusion in the entire second half of the book. It seems the protagonist is 38 years old for the vast majority, and there seems to be too much activity within that one year of life. At the very least, it's a disconnect with the pace of the first half, but it did require quite a bit of thinking on my part to ensure that yes, indeed he was 38. I still think I must have read something wrong. It's not a huge deal, but anything that breaks the continuity of a book and makes you step outside the story is not a good thing. There are also some grammatical/spelling issues in the Kindle version that should be fixed in a revision.
But, despite the above I did not rate this 4 stars. Put simply, this is a clever novel. The story itself has probably been done before: socially-awkward guy meets girl(s), miscommunicating and misunderstanding, nearly living a spinster life. What sets this one apart is the format of the story, how it paces, how the characters intertwine so perfectly (without seeming contrived), and ultimately how you feel toward the protagonist. It is a complicated story, but in the best ways.
First good point, the protagonist. Many novels might require that you just like them outright. Look how nice, how perfect, how sympathetic you should be, etc. Francis Egan, though, is complicated. Yes, you feel compassion to him, but it's equally frustrating to see how he bumbles about. The protagonist's best friend Ciss plays the role of our frustration, helping poor Francis out. By the end you will love Francis, but not because he's perfect - rather, in spite of his imperfection. To me, this is far more refreshing than setting up main characters that we're forced to like.
Second, the format. The book is introduced with chemical elements (hence, the title) that correspond to the chapter ahead. It seems contrived for the first few chapters, but eventually you start to understand not only why it is done this way, but how cleverly it fits with the story itself. It becomes almost a brilliant way to tell a story within the story, something I've not really seen done before in quite this way.
Third, the pacing. While I mentioned problems in the pacing with Francis as an adult (was he always age 38 or did I read it wrong?), the book as a whole is well paced. There were few, if any, lulls. The book takes you through Francis's entire teenage to adult life, but quickly moving to a new phase of life just before the current one is exhausted. At no point do you think, "ok, I get it, let's move on."
Fourth, the stories. Some of the stories are so absurd that it's almost like a drunk guy at a bar claiming he's done some crazy stuff. However, despite their absurdity, every story is completely believable within the narrative. I think most of us can easily identify with Francis and his inability to read signals and truly understand people, so to have these crazy stories happen to him is almost like having them happen to me, the reader. The characters of this book intertwine in situations that induce hilarity, nausea, frustration, and many other emotions. The ability to make me feel these emotions is something only really good storytellers can do. Movies add music to help with this, but a book has to me much more subtle.
Finally, the reason for 5 stars. I give 4 stars to books that I did love, and are somewhat exceptional. The 5th star is designated to such a book that I plan to reread. I think this will truly be one of those. I want to relive this story. Part of me wants a movie, but I think a movie would ruin it and turn it into a typical Rom-Com starring probably Hugh Grant or something. It's good enough left as a book.
So, thanks to the author for writing something a bit refreshing during a year that seems to be full of paranormal and YA.
Miscellanea: setting is United Kingdom, with plenty of such colloquialisms and culture references. Language is not tame, but nor is it gratuitous; several F-bombs, but not overdone or out-of-place. Sex is in the book, but it is not detailed. Recommended for adults or mature older teenagers.