Google Spreadsheets include an import features to help you copy subsheets from another spreadsheet into the currently open sheet. If you however need to merge multiple sheets, Google Scripts can help. Put them all in one folder and run a script that will create a master sheet will all the sheets pulled from other sheets.

function mergeSheets() {
  /* Retrieve the desired folder */
  var myFolder = DriveApp.getFoldersByName(SOURCE).next();

  /* Get all spreadsheets that resided on that folder */
  var spreadSheets = myFolder.getFilesByType('application/vnd.google-apps.spreadsheet');

  /* Create the new spreadsheet that you store other sheets */
  var newSpreadSheet = SpreadsheetApp.create('Merged Sheets');

  /* Iterate over the spreadsheets over the folder */
  while (spreadSheets.hasNext()) {
    var sheet = spreadSheets.next();

    /* Open the spreadsheet */
    var spreadSheet = SpreadsheetApp.openById(sheet.getId());

    /* Get all its sheets */
    for (var y in spreadSheet.getSheets()) {
      /* Copy the sheet to the new merged Spread Sheet */
      spreadSheet.getSheets()[y].copyTo(newSpreadSheet);
    }
  }
}