So yes, Susan Cooper's King of Shadows, where 20th century Shakespearean actor Nat Field's ends up in historical 16th century London as a theatre apprentice (and under none other than William Shakespeare's tutelage at that) and indeed in the at that time just newly constructed Globe Theatre (having been mysteriously transported back in time from the 20th century, well actually, having been made to switch places with a 16th century apprentice of the same name ill with the bubonic plague) is for all intents and purposes an engaging, fun, and delightfully informative historical fiction romp. For author Susan Cooper absolutely and totally knows her history, and 16th century London thus remarkably and magically comes wonderfully alive under her pen (from the sights, the sounds, even the smells of the city to how the Globe as a theatre works, how it is run and managed). And yes indeed, having Nat also speak in an Appalachian drawl, read dialect, is really and truly an ingenious narrative device and tool, considering that many linguists now surmise that standard Elizabethan English would have sounded rather similar to the dialects and parlances of Appalachia (and this factoid, it of course and naturally smoothes out potential problems of communication and comprehension, as Nat Field due to his background, due to his hailing from Appalachian Tennessee, is therefore able to for the most part easily communicate in 16th century London, whereas a more standard modern American vernacular would likely sound very strange and perhaps even incomprehensible to a 16th century Londoner and vice versa).
But although I have definitely and personally simply enjoyed and continue to much appreciate King of Shadows as basically an entertaining time travel fantasy in and of itself (and about one of my favourite playwrights), there are also and nevertheless some rather heavy and potentially saddening issues and scenarios presented and approached within Susan Copper's narrative. Because main protagonist Nat is not just a talented young American Shakespearean actor, he is also dealing with much personal sadness and trauma (grief, loss, mental anguish both in the present and also later in the past, as the warm and nurturing father figure he encounters in William Shakespeare is subsequently torn from him when he is, and definitely against his wishes and desires, simply transported back to the present time once the reasons for which he was catapulted into the past in the first place no longer exist).
Now as a child reader, or perhaps more precisely, as an older Middle Grade reader, as I believe King of Shadows is suitable from age eleven or so onwards, I would likely have simply taken and accepted the above mentioned reasons as to why Nat Field has to be switched with his namesake and Elizabethan counterpart Nathan Field as a basic and yes, even logical given (that because the historic, Elizabethan Nathan Field is ill with the bubonic plague, he might easily infect and likely kill William Shakespeare and thus destroy his literary legacy and fame). However, my adult self is of course a bit more jaded, cynical and fond of basic logistics. And if Shakespeare catching the plague from Nathan Field would kill him and thus prevent him from penning many of his most famous plays, then these plays should really by simple logic and deduction not even exist in the present day (but they in fact do, yet are deemed as threatened and in need of rescuing).
Also and finally, I do kind of find it rather hard to completely believe that when the newly returned to the present Nat Field tells his story to his friends and fellow actors Gil and Rachel, they not only IMMEDIATELY believe him, but almost AUTOMATICALLY surmise the reasons as to why Nat was sent to the past in the first place (to protect Shakespeare, or rather, his literary legacy). For yes indeed and in my opinion, it would feel a bit more authentic and realistic for Gil and Rachel to originally have entertained at least some doubts, to not have immediately captured and focused on the reasons for Nat's "voyage" to 16th century London. All in all though, Susan Cooper's King of Shadows is both a fun and solidly readable time travel adventure, perfect for young Shakespeare and history enthusiasts, and really for anyone in the mood for a delightful and engaging escape from the present into the past (into 16th century England, London and its environs).