The Legends of the Ouroboros universe includes many series and they can be read independently–you may miss small Easter eggs, but you do not have to read them all to understand the world.
You will, however, need to read all of the series to read the final series in the world: The Ouroboros Society.
Recommended Reading Order
Start with the Riftverse series (SFF/PNR whychoose) in the following order (no matter what the retailers suggest!)
Codename: The Rift
The Beast Rising Boxed Set (Pre-Codename Prequel Collection)
A New World Order
Hallelujah, Call the Magickal Midwife, & The Kitty Does Halloween (Shorts)
The Alpha & The Omega
The Beast, The Maiden, and the Perfect Present (short)
Trials of the Beast
Books 5-8 (Unannounced)
Next, start the Misfit Protection Program (PNR/Shifter/Small Town whychoose) series
Road to the Hollow (Prequel; also available in audio!)
Return to the Hollow
Roused in the Hollow (only available in Home to the Hollow omnibus)
Rejected in the Hollow
Revealed in the Hollow
Books 3-5 Unannounced
After that, start the Villains & Vixens (Contemporary/Dark/Criminal whychoose) series
Bloodthirsty
Ruthless
Books 3 & 4 Unannounced
Then, start the Triangles & Tribulations (Fantasy/Shifter/Humorous whychoose) series
Hoist the Flag (Prequel)
Yo Ho Holes
Books 3-5 Unannounced
Next, start the Faetal Attraction (Urban Fantasy/Shifter/PNR whychoose) series
Hell on Wheels
Books 2-4 Unannounced
Now, try the Vellas (PNR whychoose University)
Blood on the Ice
Veiled Flame
Coming Later, Coven of the Serpent and Exile Islands series!