#SOGL, 64 bytes be%→M"Q└ƨS‘ač;┼→S%‘A |e3 -* |++M?tMSeM-9*@*a+┼Ot}be÷:?{teSa┼Ot Explanation: First function:

   →M  define function M which pushes
b      the book amount
  %    mod
 e     the bookshelf width

second function:

           →S  create function S (example input: 3)          [3]
"Q└ƨS‘         push the string "|||XXX|||" (the book)        [3, "|||XXX|||"]
      *        multiply by the number on stack (book count)  ["|||XXX||||||XXX||||||XXX|||"]
       a       push variable A (later defined "|||")         ["|||XXX||||||XXX||||||XXX|||", "|||"]
        č      chop into char array                          ["|||XXX||||||XXX||||||XXX|||", ["|", "|", "|"]]
         ;     swap top 2 on stack                           [["|", "|", "|"], "|||XXX||||||XXX||||||XXX|||"]
          ┼    horizontally append                           [["||X||X||X|", "||X||X||X|", "||X||X||X|"]]

this function expects a number (book count) on stack and outputs the bookshelfs books

["||X||X||X|",
 "||X||X||X|",
 "||X||X||X|"]

Further down example given is e=3 (bookshelf width) and b=8 (book amount)

%‘A              var A = "|||"                        
    |            push "|"                      ["|"]                
     e3*         push E * 3                    ["|", 9]             
         -*      push that many "-"es          ["|", "---------"]   
            |+   append "|"                    ["|", "---------|"]  
              +  prepend the "|"               ["|---------|"]      

this is the bookshelf top/bottom line and always stays on the stack first part (half-empty bookshelf)

First main part

M?               }               if the modulo != 0
  tM                             output the bookshelf top/bottom line
    S                            execute the S function width the modulo
     eM-                         push bookshelf width - modulo (empty space count)
        9*                       multiply by 9 (books are 3x3 so 3x3 spaces)
          @*                     get that many spaces
            a+                   append to that "|||"
              ┼                  horizontally append
               O                 output
                t                output the bookshelf top/bottom line

And the last part

be÷            floor divide book amout by width (full shelves)
   :?          if not 0 (a bug makes all loops execute once)
     {         repeat
      t        output the bookshelf top/bottom line
       eS      execute S with shelf width (full shelf)
         a┼    horizontally append "|||"
           O   output
            t  output the bookshelf top/bottom line