How do you convert nginx equivalent URL rewrite for an additional:

RewriteRule ^read/([0-9]+)/?$ /read/?u=$1 [QSA,L]

Provide the carrying out a try:

if (!-e $request_filename) {
    rewrite ^read/([0-9]+)/$ /read/?u=$1 permanent;
}

This can be done in 2 ways, having a location:

# the ?<u> assigns the capture to $u.  Some older pcres need ?P<u>
location ^/read/(?<u>[0-9]+)/?$ {
  rewrite ^ /read/?u=$u last;
}

or with only a rewrite:

rewrite ^/read/([0-9]+)/?$ /read/?u=$1 last;

nginx will append the query string automatically (you are able to disable the behaviour with the addition of another ? in the finish from the rewrite target).