[1] Sir Thursday continues from the preceding book, following the adventures of a boy named Arthur as he attempts to claim mastership of the fourth part of a magical world.

While Leaf is able to pass through the front door and return to Earth, the presence of a magical doppelgänger of Arthur prevents him from doing so, and he is forced to remain in the House.

Leaf infiltrates the hospital, seeking to obtain and destroy the magical object used to create the Skinless Boy: a pocket torn from one of Arthur's shirts.

As with the previous books in the series, the themes of seven, sin and virtue are prevalent throughout Sir Thursday providing the personality characteristics for many of the main characters.

[2] In Sir Thursday, readers are also introduced to the power issues that Arthur begins to struggle with – his natural compassion and desire for fairness and kindness, against the characteristics displayed by Dame Primus and other House creatures, and the influence of the 'Keys', the powerful magical objects that he acquires throughout the series.

It is ranked 4 stars by Amazon users,[9] and The Times stated "Nix's imagination is matched only by his prose style.

VOYA says that the book "..is well written, action packed, imaginative, and full of quirky memorable characters...",[11] and Children's Literature agrees that "It is a readable choice..."[12] The School Library Journal also extensively praised the book, saying "Reluctant or 'hard to fit' readers will find this series enjoyable, as Nix is able to create vivid pictures in the minds of his readers.

The most common complaint was on the speed of the plot – the School Library Journal summarises all the comments, saying that "the events move a little slower than in previous installments..."[13] – and the difficulty in entering the series without reading all the previous books first: VOYA simply says "readers attempting to enter the series through this volume are likely to be quickly lost.

"[11] Children's Literature also criticised the repetition of previous content, saying there is "..entirely too much front matter [which] sags with the necessity of replaying earlier story lines.