What is the order of the Warriors book series?

You should start with the original series first. This is the order from the first one to the last one:

Into the Wild

Fire and Ice

Forest of Secrets

Rising Storm

A Dangerous Path

The Darkest Hour

Then you should read Bluestar's Prophecy and Firestar's Quest.

Next you should read the New Prophecy series.

Midnight

Moonlight

Dawn

Starlight

Twilight

Sunset

Then read the Power of Three series:

The Sight

Dark River

Outcast

Eclipse

Long Shadows

Sunrise

Then finally the Omen of the Stars series:

The Fourth Apprentice

Fading Echoes

Night Whispers (to come out soon)