Distinction between primary and secondary sources varies somewhat between disciplines. This is especially evident when contrasting history with English literary studies.
In literary studies, the item (novel, play, story, image) itself is a primary source. Other documents created at the time of publication are generally considered to be secondary sources: contemporary book reviews and author profiles and interviews found in newspapers and magazines.
Contemporary publisher's catalogues and advertisements in other publications may provide insight into the history and reception of a book. This kind of material was often regarded as ephemeral and rarely saved, Some authors were meticulous at scrapbooking and keeping records that may be found in archival resources including online exhibitions and digital collections. Likewise, some publishers kept good records of sales but these may not be open to the public or otherwise hard to access, even if they are in public research institutions. An unpublished account of sales would be considered a primary source. To find author or publisher archives, try Googling the name with the word archive.
Search the library catalogue using the author's name in a subject search.
Look for entries with the name of the author by itself, or with subdivisions such as --Correspondence; -- Diaries; --Notebooks, sketchbooks, etc.
Alternately, try a keyword search using the author's name and a word such as: diaries, journals, correspondence, letters
example personal name as subject search: beardsley aubrey
Interviews may be indexed in the "search everything" tool since it is designed to search digital content in addition to the library's book and media catalogue.
Since the creators of the Yellow Book are long dead, interviews with them are likely to be found, if at all, in contemporary publications issued during their lifetimes. You may find material in the archival journal and newspaper databases identified on the Journal & Article Resources page of this guide, Most of the periodical indexing tools including MLA International Bibliography (started in 1926) are too recent to be of great value. You may have some luck searching C19. Instances of posthumous publication of letters, interviews and other archival material may be indexed in more recent finding aids.
Googling the author, illustrator or publisher and the word archive will help you locate physical collections containing archival material pertaining to the person or corporate entity. If you're lucky, you may find that some of the items have been digitized. Also, scholarly digital websites can contain very useful material.
example found Googling: "kelmscott press" archive, is the following section of a scholarly website called William Morris Archive, under the Book Arts folder: The Kelmscott Press