Original levels of canon
The Star Wars wikia page on dual-phase lightsabers list 6 novels that the lightsabers appear in. Under Star Wars levels of canon this would be C-Canon (or Continuity canon).
C-canon is Continuity Canon, consisting of all recent works (and many older works) released under the name of Star Wars: books, comics, games, cartoons, non-theatrical films, and more.
This is generally considered to be the third level of canon; the first being G-canon and the second being T-canon.
G-canon is George Lucas Canon; the six Episodes and any statements by George Lucas (including unpublished production notes from him or his production department that are never seen by the public). Elements originating with Lucas in the movie novelizations, reference books, and other sources are also G-canon, though anything created by the authors of those sources is C-canon.
T-canon, or Television Canon, refers to the canon level comprising the feature film Star Wars: The Clone Wars and the two television shows Star Wars: The Clone Wars and the Star Wars live-action TV series. It was devised recently in order to define a status above the C-Level canon
New Disney revision
As pointed out by Darth Satan {SFF.SE user, not canon character) the Disney revisions have changed what is considered canon.
On April 25, 2014, Lucasfilm and Disney revised the franchise's canon. They announced that the existing six films and The Clone Wars television series are the "immovable objects" of Star Wars storytelling. Previously published material has been relabeled under a "Legends" label, and future content will present a different vision of people, places and events after Return of the Jedi. They also announced that all future Star Wars stories will be connected, with guidance coming from the Star Wars story group.
From Wikipedia citing the Star Wars website.
So under the Disney revisions they wouldn't be considered true canon, and would only exist under a "Legends" label.
To summarise: if you go by the original levels of canon then they are C-canon, if you go by the new Disney revisions then they aren't true canon and just exist as Star Wars Legend.