Here is a list of all cannon couples, in order of which book they appear in. The three at the bottom were confirmed in Wands and Worlds interviews with the authors. If you notice any mistakes, please let me know!

Into the Wild:
Firestar x Spottedleaf

Fire and Ice:
Graystripe x Silverstream
Yellowfang x Raggedstar

Forest of Secrets:
Bluestar x Oakheart
Thistleclaw x Tailtip
Tigerstar x Goldenflower

Rising Storm:
Firestar x Sandstorm

A Dangerous Path:
Cloudtail x Brightheart
Dustpelt x Ferncloud
Whitestorm x Willowpelt

Midnight:
Squirrelfight x Brambleclaw

Moonrise:
Tigerstar x Sasha
Crowfeather x Feathertail
Stormfur x Brook

Starlight:
Crowfeather x Leafpool
Squirrelflight x Ashfur

Twilight:
Daisy x Smokey
Smoky x Floss

Sunset:
Daisy x Cloudtail

The Sight:
Crowfeather x Nightcloud
Graystripe x Millie

Firestar’s Quest:
Cloudstar x Birdflight
Petalnose x Rainfur

The Dark River:
Lionpaw x Heatherpaw

The Warrior's Refuge:
Moss x Husker

Interviews:
Cinderpelt x Firestar
Dustpelt x Sandstorm
Tawnypelt x Rowanclaw
Daisy x Spiderleg