ISHN <- read_html("http://....")

map_df(1:63, function(i) {

 pg <- read_html(sprintf(ISHN, i))

 data.frame(ISHN_title=html_text(html_nodes(pg, ".title a")),
         ISHN_country_year=html_text(html_nodes(pg, ".study-country")),
         ISHN_author=html_text(html_nodes(pg, ".sub-title div")),
         stringsAsFactors=FALSE)

}) -> ISHN_df

Error in sprintf(ISHN, i) : 'fmt' is not a character vector