There's a lot of words in the English language, and only so many of them fit on a single page. Therefore, this tool is divided into Volumes.
Volume 1 is for one-letter words.
Volume 2 is for two-letter words.
Volume 3 is for three-letter words.
And so on.
Additionally, the Volumes themselves may be further divided between common and uncommon words, as most public available word lists provide only the top so-many words for any given classification. Uncommon words are also less likely to appear in any given cryptogram, so these suggestions on the whole are less likely to be fruitful.