Working through the book, you find how variable it is.
Too many entries refer back to other entries, and there's seldom an indication as to the class and period in which a term was current. The book starts strongly with supplementary detail, but this peters out.
Too often it falls back on 'explaining' rhyming-slang, and ignores other forms of London slang, that have persisted. Jokey references to German sausages are fine, but did they refer to Queen Victoria's choice of husband, or to her daughters' chosen husbands. And do they count as 'slang' in Victorian London?
Probably, 'Victorian' is too long a period to cover, especially for London... 1837 to 1901 is a big task. Imagine trying to cover the last 64 years of London slang... I doubt it'd be possible.
As it is, this book may be of use to you if you're puzzled by something written by a Victorian author using contemporary London slang, and draws on sources such as Henry Mayhew, but you can't rely on it.