fix the issue reported at https://stackoverflow.com/q/56061122/559676:…

… when format is NULL, I shouldn't call files2[[format]]; format will be NULL if the output format is not HTML or LaTeX, in which case I'll just use the default list of Rmd input files

DESCRIPTION

@@ -1,7 +1,7 @@
 Package: bookdown
 Type: Package
 Title: Authoring Books and Technical Documents with R Markdown
-Version: 0.9.15
+Version: 0.9.16
 Authors@R: c(
     person("Yihui", "Xie", role = c("aut", "cre"), email = "xie@yihui.name", comment = c(ORCID = "0000-0003-0645-5666")),
     person("JJ", "Allaire", role = "ctb"),

NEWS.md

@@ -22,6 +22,8 @@
 
 - Hyphenated words will be correctly highlighted in the search results now if spaces are used in the search keyword instead of dashes, e.g., you can search for `hand-off` using the keyword `hand off` (thanks, @aronatkins, #701).
 
+- When `rmd_files` is configured in `_bookdown.yml`, `render_book()` will fail if the output format is not HTML or LaTeX (thanks, Ladislas Nalborczyk, https://stackoverflow.com/q/56061122/559676).
+
 ## MINOR CHANGES
 
 - Added alt/hover text to icons on the `gitbook` toolbar (thanks, @aronatkins, #698).

R/utils.R

@@ -64,10 +64,8 @@ source_files = function(format = NULL, config = load_config(), all = FALSE) {
     if (is.character(subdir)) subdir else '.', '[.]Rmd$', ignore.case = TRUE,
     recursive = subdir_yes, full.names = subdir_yes
   ))
-  if (length(files2 <- config[['rmd_files']]) > 0) {
-    if (is.list(files2)) {
-      files2 = if (all && is.null(format)) unlist(files2) else files2[[format]]
-    }
+  if (length(files2 <- config[['rmd_files']]) > 0 && !is.null(format)) {
+    if (is.list(files2)) files2 = if (all) unlist(files2) else files2[[format]]
     files = if (subdir_yes) c(files2, files) else files2
   } else {
     files = files[grep('^[^_]', basename(files))]  # exclude those start with _