The base ball “fan” (sports enthusiast) has been cited in print from Kansas City, Missouri, in May 1885. “Fandom” is the world (kingdom) of fans.
 
“A fanless summer is not to be thought of; no more is the purchase of these dainty breeze makers anywhere except in the capital of Fandom—Our store”—referring to wind fans—was printed in the Burlington (VT) Independent on June 23, 1894.
 
The earliest citations of “fandom” are in baseball. “The best men of the Western are being rapidly signed by the big clubs, and there will be mourning in fandom as a consequence” was printed in the Sioux City (IA) Journal on September 22, 1894.
 
 
Wikipedia: Fandom
A fandom is a subculture composed of fans characterized by a feeling of empathy and camaraderie with others who share a common interest. Fans typically are interested in even minor details of the object(s) of their fandom and spend a significant portion of their time and energy involved with their interest, often as a part of a social network with particular practices (a fandom); this is what differentiates “fannish” (fandom-affiliated) fans from those with only a casual interest.
 
A fandom can grow around any area of human interest or activity. The subject of fan interest can be narrowly defined, focused on something like an individual celebrity, or more widely defined, encompassing entire hobbies, genres or fashions. While it is now used to apply to groups of people fascinated with any subject, the term has its roots in those with an enthusiastic appreciation for sports. Merriam-Webster’s dictionary traces the usage of the term back as far as 1903.
 
Fandom as a term can also be used in a broad sense to refer to the interconnected social networks of individual fandoms, many of which overlap.
 
(Oxford English Dictionary)
fandom, n.
orig. U.S.
The world of enthusiasts for some amusement or for some artist; also in extended use.
1903   Cincinnati Enquirer 2 Jan. 3/1 (heading)  Fandom puzzled over Johnsonian statements.
     
Newspapers.com
23 June 1894, Burlington (VT) Independent, pg. 5, col. 1 ad:
FANS AT THE BEE HIVE.
(...)
A fanless summer is not to be thought of; no more is the purchase of these dainty breeze makers anywhere except in the capital of Fandom—Our store.
N. E. Ghamberlin.
   
Newspapers.com
22 September 1894, Sioux City (IA) Journal, pg. 2, col. 1:
Watkins to Go to Pittsburg.
INDIANAPOLIS, Sept. 5L (? -ed.).—The National league is making sad inroads upon the playing talent of the Western league clubs. The best men of the Western are being rapidly signed by the big clubs, and there will be mourning in fandom as a consequence. 
 
8 June 1895, Cincinnati (OH) Post, pg. 4, col. 3:
EWING IS MAD.
An Epistle Dedicated to the Inhabitants of Fandom.
Gossipy Echoes from the Camp of the Reds.
 
Newspapers.com
14 August 1895, The Daily Inter Ocean (Chicago, IL), pg. 4, col. 1:
EMPERORS OF FANDOM
Baseball Magnates of the National League in Session.
 
14 March 1896, Scranton (PA) Tribune, “Our Farmer-Bred Players,” pg. 8, col. 5:
Where do the best ball players come from?
 
That’s a question sufficiently knotty to keep Fandom puzzled for a week of Sundays.
     
OCLC WorldCat record
PSFS News: the gazette of Philadelphia fandom.
Author: Oswald Train
Publisher: Philadelphia, Oswald Train, 1945.
Edition/Format:   Print book : English
 
OCLC WorldCat record
Science fiction fandom
Author: Joseph L Sanders
Publisher: Westport, Conn. : Greenwood Press, 1994.
Series: Contributions to the study of science fiction and fantasy, no. 62.
Edition/Format:   Print book : English
Summary:
This work surveys science fiction fandom’s history, manifestations and accomplishments, including clubs, fanzines and conventions. The 24 essays are divided into sections considering topics including the types of people who become fans, and social interactions in the form of local clubs