San Jose refers to Plaza de San Jose, one of several plazas in San Juan, which I believe is an old section of San Juan still popular with the arts community.
Libreria Campos was a San Juan publisher and bookseller for whom I can find publications dating from the 1930s to 1950s. They must have either sponsored or hosted the 1955 fair. Their history before and after the date range above is unknown to me.
"Allen Esq" offers a clue to the history of the fair also, but I can't, as yet, connect it to anything specific.
So, for now, this little bookmark is the only relic I can find online of that book fair nearly 60 years ago in San Juan, Puerto Rico.