Equivalent of cftchapternumwidth when using book class?

Use the tocloft package.

documentclass{book}
usepackage{tocloft}
setlength{cftbeforechapskip}{0.5em}
setlength{cftchapnumwidth}{10em}

Here is a minimal example using memoir:

documentclass{memoir}

setlength{cftbeforechapterskip}{0.5em}
setlength{cftchapternumwidth}{10em}

begin{document}

tableofcontents

chapter{A chapter}
section{A section}

chapter{Another chapter}

end{document}

---

Here is a replication of the output using tocloft in the book document class (note that chapter-level lengths for the ToC under tocloft uses cftchap..., not cftchapter... as is used with memoir):

documentclass{book}

usepackage{tocloft}

setlength{cftbeforechapskip}{0.5em}
setlength{cftchapnumwidth}{10em}

begin{document}

clearpage
addcontentsline{toc}{chapter}{contentsname}
tableofcontents

chapter{A chapter}
section{A section}

chapter{Another chapter}

end{document}

---

Without any packages, you'll have to change the definition of l@chapter. Here's the original definition from book.cls:

newcommand*l@chapter[2]{%
  ifnum c@tocdepth >m@ne
    addpenalty{-@highpenalty}%
    vskip 1.0em @plusp@
    setlength@tempdima{1.5em}%
    begingroup
      parindent z@ rightskip @pnumwidth
      parfillskip -@pnumwidth
      leavevmode bfseries
      advanceleftskip@tempdima
      hskip -leftskip
      #1nobreakhfil
      nobreakhb@xt@@pnumwidth{hss #2%
                                 kern-p@kernp@}par
      penalty@highpenalty
    endgroup
  fi}

Here's the updated definition to replicate the behaviour of memoir and/or tocloft:

documentclass{book}

makeatletter
renewcommand*l@chapter[2]{%
  ifnum c@tocdepth >m@ne
    addpenalty{-@highpenalty}%
    vskip 0.5em% <----------------- Similar to cftbeforechapskip from tocloft
    setlength@tempdima{10em}% <--- Similar to cftchapnumwidth from tocloft
    begingroup
      parindent z@ rightskip @pnumwidth
      parfillskip -@pnumwidth
      leavevmode bfseries
      advanceleftskip@tempdima
      hskip -leftskip
      #1nobreakhfil
      nobreakhb@xt@@pnumwidth{hss #2%
                                 kern-p@kernp@}par
      penalty@highpenalty
    endgroup
  fi}
makeatother

begin{document}

clearpage
addcontentsline{toc}{chapter}{contentsname}
tableofcontents

chapter{A chapter}
section{A section}

chapter{Another chapter}

end{document}