Simple key-value example

Here's an implementation with expl3 keys.

documentclass{article}
usepackage{xparse}

ExplSyntaxOn

% define the keys
keys_define:nn { fauve/books }
 {
  title  .tl_set:N = l__fauve_books_title_tl, % the title
  author .tl_set:N = l__fauve_books_author_tl, % the author
 }

% formatting
NewDocumentCommand{mycommand}{m}
 {
  group_begin:
  keys_set:nn { fauve/books } { #1 } % populate the keys

  % format the book
  paraddvspace{topsep}
  fauve_books_author:V l__fauve_books_author_tl
  fauve_books_title:V l__fauve_books_title_tl
  paraddvspace{topsep}
  group_end:
 }

cs_new_protected:Nn fauve_books_title:n
 {
  tl_if_empty:nF { #1 }
   {
    parnoindenttextbf{Title:}~#1
   }
 }
cs_new_protected:Nn fauve_books_author:n
 {
  tl_if_empty:nF { #1 }
   {
    parnoindenttextbf{Writer:}~#1
   }
 }
cs_generate_variant:Nn fauve_books_title:n  {V}
cs_generate_variant:Nn fauve_books_author:n {V}

ExplSyntaxOff

begin{document}

mycommand{title=The new book, author=John Doe}

mycommand{title=Another book}

end{document}

I have separated the formatting commands specific for each key, but that's optional.

FWIW, I'll show how to write such a macro in ConTeXt. ConTeXt has a few key-value driven interfaces, but for something as simple as this, I would just piggyback oon the setupvariables[...][..=..] and getvariable{...}{...}.

unprotect
definemycommand
    {dosingleargumentmycommand_setup}

defmycommand_setup[#1]%
    {bgroup
     setvariables[mycommand][#1]%
     mycommand_process
     egroup}

defmycommand_process
    {startlines
     doifsomething{getvariable{mycommand}{author}}
        {bold{Writer:} getvariable{mycommand}{author}endgraf}%
     doifsomething{getvariable{mycommand}{title}}
        {bold{Title:} getvariable{mycommand}{title}}%
     stoplines}
protect

starttext
mycommand[title={The new book}, author={John Doe}]

mycommand[title={The new book}]
stoptext

Note that I modified the interface (use mycommand[...] instead of mycommand{...}) to match the ConTeXt convention that arguments in square brackets are for options and arguments in curly brackets are for text which is typeset.

With expkv-def you can use data and dataT keys.

A data key will store the information you provide in the value in a macro, but this macro also takes two arguments. If the key wasn't used it'd execute its second argument, while if the key was used it'd execute its first argument and append the value given to it in braces (so as an argument to that code).

A dataT key behaves similar, but the macro will only take one argument and would do nothing if the key wasn't used yet, whereas it would execute its argument and append the given value if it was used.

Just storing the value in a variable would be possible with the store key type instead of data.

documentclass[]{article}

usepackage{expkv-def}

makeatletter
ekvdefinekeys{fauve}
  {
     data  title  = fauve@title
    ,dataT author = fauve@author
    ,dataT year   = fauve@year
  }
ekvsetdeffauve@set{fauve}
newcommandmycommand[1]
  {%
    begingroup
      fauve@set{#1}%
      fauve@author{parnoindenttextbf{Writer} }%
      fauve@title
        {parnoindenttextbf{Title} }%
        {parnoindentemph{No title}}%
      fauve@year{parnoindenttextbf{Year} }%
    endgroup
  }
makeatother

begin{document}
mycommand{title=The new book,author=John Doe}
bigskip
mycommand{title=The new book}
bigskip
mycommand{title=The new book,year=2020}
bigskip
mycommand{author=John Doe,year=2020}
end{document}