It is very hard to summarize/explain the Thursday Next books because they are wonderfully bizarre, but I will do my best. Honestly, though, if you want to start the series (and you should!) start at the beginning (The Eyre Affair) to get the full effect. Something Rotten and First Among Sequels are the fourth and fifth in the series, respectively.
Thursday Next lives in an alternate future from ours, a version of Britain where the Crimean War has raged for a century, cheese has been made illegal, and a police force exists to monitor and enforce rules in the literary world (which sometimes bleeds into ours). Thursday is a Jurisfiction officer, and capable of jumping into books and interacting with characters. Sometimes these characters invade our world as well.
In Something Rotten, Thursday is hell-bent on finding her husband, Landen, who was eradicated by the worldwide Goliath Corporation in a previous novel. She’s the only person who remembers his existence, even though she has a two year old by him. She’s also brought Hamlet into the real world with her, in order to solve some problems with his play AND someone keeps trying to kill her. First Among Sequels is actually the first in a second series about Next. It’s set about 14 years in the future, and focuses more on her interacting with two different literary versions of herself, written about her legendary exploits. I really don’t want to give any more away.
These books are very funny, and definitely the kind of humor where you can find something new each time you read them. Lots of puns, meta-references (First Among Sequels is VERY meta) and general silliness. I liked Something Rotten a bit more–it was probably my favorite Next novel yet. There’s a lot going on, and the plot moves very quickly. Overall, both were good additions to the series.