Is it permissible to request a symbolic sum of money in exchange for sending the remaining parts of a book being authored, some of which has been published for free online, with the remaining parts to be sent as installments or at spaced intervals?

There is no objection to displaying samples of your work to market and sell it at the agreed-upon price. There is also no objection to delivering the book in parts if that is mutually agreed upon. It is permissible to protect copyrights by following legal procedures, as intellectual property rights are safeguarded by Islamic law.