What is the correct term to describe literary works that are only partly fictional?

Semi-fiction and semi-fictional don't seem to be unusual terms. I would use them.

Semi-fiction is fiction implementing a great deal of non-fiction, for example: a fictional depiction "based on a true story", or a fictionalized account, or a reconstructed biography. Often, even when the author claims the story is true, there may be significant additions and subtractions from the true story to make it more suitable for storytelling. One such example would be Tim O'Brien's The Things They Carried. Wikipedia

from Google Books:

• While semi-fictional text can emerge in a directed way, we also highlight...
• Gathered here are some of Dreiser's best pieces of short fiction and semi-fiction.
• Metamorphosis: A Small Piece of Semi-fiction.
• The Retreat - A Semi-Fictional Memoir Exploring Common ...
• It is in any case a semi-fictional genre, novelistic in its own way.

The Da Vinci Code is a secret history.

Secret history is ... used to describe a type or genre of fiction which portrays a substantially different motivation or backstory from established historical events.

John Grisham's books (at least the ones I've read) are not secret histories. Most fiction (unlike Harry Potter, LOTR) is set in the real world, and accurately depicting certain elements from the real world as background is very common in fiction. John Grisham's thrillers often go into details of the law and the legal profession more extensively than most fiction about lawyers, but this is just one point on a continuum, and I don't know of a special word for this.

Anything that is not a completely true account is fiction.

Maybe there is some sub genre of fiction (things like historical fiction, for example--fictional stories that take place in a historically accurate setting) that would satisfy you but, there's no "almost fiction." It's simply true or not true.

Fiction is fiction. Different eras and different audiences vary widely in the sort of background they demand, but every storyteller has to supply some measure of what W.S. Gilbert called "merely corroborative detail intended to provide artistic verisimilitude to an otherwise bald and unconvincing narrative." Verisimilitude may reside in familiar quotidian detail or romantic "history" or in the depiction of motive and mental states; but without some such detail there's no framework to set the story in and no identifiable connection with the reader's own experience.

How about faction?

noun, Informal.

1. a form of writing or film making that treats real people or events as if they were fictional or uses them as an integral part of a fictional account.
2. a novel, film, play, or other presentation in this form.

Origin 1965-70; blend of fact and fiction

[ I know - it's a horrible portmanteau. ]

From the perspective of history, historic fiction might fit your description.

What about Roman à clef.

• a novel in which real persons or actual events figure under disguise. (MW)

French for novel with a key, is a novel about real life, overlaid with a façade of fiction. The fictitious names in the novel represent real people, and the "key" is the relationship between the nonfiction and the fiction.

(Wiki)

Pseudo-fiction is a coined term for this genre but it is not well-established yet. The term is also used to describe Dan Brown's The DaVinci Code in various reviews.

A plethora of books/booklets have come out in response to Brown’s work of pseudo-fiction (I say that because he has come to believe his own fiction, and clearly the book is intended to communicate an underlying matrix of “facts” even while presenting them in an allegedly fictional milieu)...

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I read Dan Brown's claim of fiction as a double negative. The first negative is the "fact" page which has very little basis in fact. The second negative is the claim that Dan Brown intended to write fiction. He has said on an ABC interview that if he had written the book as non-fiction he would have changed none of the details of the conspiracy and historical interpretation.

This makes it hard to classify the genre of DaCode, but I propose we create a new genre for DaCode and its ilk: pseudo-fiction. A pseudo-fiction is "false-false." It is false fiction; which means, according to the rule of double negative, it is non-fiction. Note that I did not say it was true or fact. We have no such category when it comes to literature, unless you include reference materials. However, reference materials and non-fiction works are subject to scrutiny.

The point of proposing the "pseudo-fiction" genre is to get around the claim of Brown and others that DaCode is "just fiction." It isn't fair to simply claim that something is fiction when it pretends to be more.

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The term appears in goodreads.com as well but there are a few books listed. A notable book mentioned is The Boy in the Striped Pajamas which is based on historical events (that are not actually documented) but also criticized as being unrealistic. Another genre mentioned is historical fiction for this book.

Another book that is mentioned as a pseudo-fiction is Daniel Defoe's A Journal of the Plague Year which is a fictionalised account of one man's experiences of the year 1665, in which the Great Plague struck the city of London.^[wikipedia]

The genre is mentioned in the book New Perspectives on Robert Graves (edited by Patrick J. Quinn) when being compared to Graves' work:

As other answerers have stated, the works you list are entirely fictional. Fiction, in general, borrows from the real world. I would challenge you to name a single work of fiction that borrows nothing from the real world.

That said,

Creative nonfiction (also known as literary nonfiction or narrative nonfiction) is a genre of writing that uses literary styles and techniques to create factually accurate narratives.

Source.

I believe the genre you want is "Docufiction" or "Historical Fiction".

Probably just historical fiction or realistic fiction. I'm not sure there is a genre name for being highly detailed. After all, most fiction is set roughly in the "real world" the fiction that is set in purely make believe worlds is its own genre, namely fantasy. Being set in a plausible real world setting isn't a genre set apart from mainstream fiction.