I spend a while to go through the document and code to figure this out.

An expression like $..book[?(@.title='"Sword of Honour"')] will not work.

It should be something like $..book[?(@.title=='"Sword of Honour"')] or $..book[?(@.title==='"Sword of Honour"')] like in JavaScript. Not sure if this is obvious, but I suggest to add it to the document since it is different from XPath syntax. Thanks.