The Original Series Reading List lacks a main ongoing storyline that is the hallmark of the other series'
literary continuities. However, some prominent storylines and tales can be
picked out. The majority of The Original Series novels are stand-alone, and most can
easily be read in any order. Accordingly there are no obvious "jumping on points" which the other series can be said to have.
Many The Original Series stories in the last two decades have included elements of
the wider Lit-verse, so many novels, short stories, comics, and audiobooks are connected due to various references in other books, or to other books. These references are mentioned in the notes.
Nine other short stories or novellas are also connected: five from Enterprise Logs (which shares references among several connected novels), two from Deep Space Nine: The Lives of Dax (which ties into the Deep Space Nine Post-Finale Series), one from The Brave and the Bold duology (which ties into the Gorkon series), and one from Seven Deadly Sins (which ties into three Lit-verse series.)
Five novels and one ebook in other series cross over with The Original Series: Discovery: Drastic Measures, Discovery: Desperate Hours, Department of Temporal Investigations: Forgotten History, Vanguard: Harbinger, Vanguard: Storming Heaven, and Vanguard: In Tempest's Wake.
Also included are the stories from Constellations (one story of which is referenced in The Next Generation: Before Dishonor), and Mere Anarchy (which references several other stories.) Various references in parts of the Crew series, the Untold Voyages series, New Visions: Issue #5, the Year Five series, the Wildstorm Star Trek Special, the DC Volume 2 Special #2, and the anthology New Worlds, New Civilizations connect The Original Series stories from those works.
The Original Series Reading List stops at the disappearance of James T. Kirk aboard the Enterprise-B and all storylines transfer to "The Lost Era", the Early 24th Century Reading List.
In addition there are the novels that comprised the
"Original Lit-verse" of the 1980s. This was a group of stories which
were eventually linked together through references and shared
characters. Of particular importance to the later Lit-verse are the Rihannsu series and The Final Reflection, many
elements of which have been more or less incorporated into the modern
continuity, though undoubtedly requiring a certain amount of retconning.
The earliest book eventually connected to the "Original Lit-verse" was
published in 1977. The continuity flourished throughout the 1980s under
editor Dave Stern. Around 1988, under the direction of Gene
Roddenberry and Richard Arnold, a push was made to cut back on shared
characters and references between novels. This was not a hard rule, and
references between books and between comics continued at a slower pace
through the mid 1990s under Stern and Kevin Ryan. I place the end of
this era, and the advent of the modern Lit-verse, around 1995/1996,
under editor John Ordover.
(Note that an explanation for my order of episodes and novels during the Five Year Mission can be seen here: The Five Year Mission: Month-By-Month. This is a very specific order that does not always match production order or airdate.)
(Note that the covers of many The Original Series books contain Enterprise designs or Starfleet uniforms which do not match the era of the book, and which sometimes do not even match each other. These should be disregarded.)