If the serialized one is in the public domain and a book isn't, then your work might be seen as derivative of the book anyway.
For instance the recent Wicked movie (and book, and play, etc) can't use ruby red slippers, because that was a detail only in the relativley more recent movie, so even though the original wizard of oz book has been in the public domain a while, some details are siloed off. I think the Holmes stories had disputes like this for a while too, where if you mentioned certain side characters it might infringe a more recent adaptation or etc.
I can see how having the public domain source material might be necessary to defend yourself against claims of infringement of the non-public domain works. That makes sense to me.
For instance the recent Wicked movie (and book, and play, etc) can't use ruby red slippers, because that was a detail only in the relativley more recent movie, so even though the original wizard of oz book has been in the public domain a while, some details are siloed off. I think the Holmes stories had disputes like this for a while too, where if you mentioned certain side characters it might infringe a more recent adaptation or etc.