The best replacement for workbook is worksheet.

The best alternatives to workbook are rated as following:
★★★★★  worksheet
★★★  spreadsheet
★★★  worksheet group
★★  workbook
★★  agency access
★★  worksheet groups
★★  excel worksheet
★  sheet
★  sheet excel
★  link data
★  hssfworkbook
★  template
★  all macros may be disabled
  work book
  fretboard logic
  worksheet excel
  textbook
  shared workspace

Your alternative for workbook is not listed? Suggest!
Speaking about workbook, one probably shouldn't forget about vba.

Other search terms related to workbook and their ratings are:
★★★★★ vba
★★★ macro
You have in mind a companion for workbook and it is not yet here? Propose!



Google Trends

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workbook
worksheet
spreadsheet