Here's my count of Book 6 material:

Spoiler: Book 6 Print/PDF-Exclusive Material
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Title Page
Thor

Copyright Page
Thirden (?)
(We don't count the ads for the other books as it may change in future prints)

Preface
Haley Starshine

Introduction p1
Rich Burlew

Introduction p2
This is is a weird one where we see bits of #421, #777, and #937.
From #421 we see some of the crying troops from the penultimate panel.
From #777 we see the guard with the clipboard, the crowd of gladiators, the 3 speech balloons from the 3 guys named Spartacus, and part of the speech balloons from Gannji and Enor in the panel below.
From #937 we see Julio, Roy, and Bandana, as well as part of the doctor's speech balloon.

Introduction p3
Roy Greenhilt

Recap p1
Belkar Bitterleaf
Durkon Allotrope Thundershield
"Durkon"*
"Malack" the Lizardfolk
Nale
Roy Greenhilt
Snarl
Tarquin

Recap p2
Durkon Allotrope Thundershield
"Durkon"*
Hel
"Malack" the Lizardfolk
Nale

Round 3 title page
High Priest of Balder
High Priest of Vafthrudnir
High Priestess of Freya
High Priestess of Hoder
High Priestess of Odin
High Priestess of Sif
High Priestess Rubyrock
Representative of Southern Pantheon

Round 4 commentary
Monster in the Darkness
Oona

New panels inserted into the second page of #1051
(This is another weird one since it's 5 panels inserted into an existing strip, and the 5th isn't even continuous from the others, but it should still count going by how we counted similar bonus material in the past.)
Beatrix aka Bandana Secundus
Blackwing
Possibly the helmed frost giant from #1050 and the first page of #1051
Roy Greenhilt
Unknown Mechane crewman (might be unique to this panel?)

Round 6 title page
Durkon Allotrope Thundersthield
"Durkon"* (no dialogue but appears in his Negative Energy spirit form so it counts)

Round 8 title page
Council of Elders Relief

Round 9 title page
Durkon Allotrope Thundershield
Logann
Shirra Copperbottom
Shirra's Sons
Sigdi Thundershield

Final page
Redcloak
Xykon

#960a
Banjo
Carol
Elan the Bard
Eliza
Felix
Kwesi
Mechane Crewman with eyepatch and pegleg
Mechane Crewman with five o'clock shadow
(I think the butterfly and other 2 Mechane crew are unique to this page)

#966a
Beatrix aka Bandana Secundus
Elan the Bard
Haley Starshine

#967a
Durkon Allotrope Thundershield
Roy Greenhilt

#967b
Belkar Bitterleaf
Mr. Scruffy
(I think the gnomes from this and the previous 2 bonus strips are unique to each page)

#989a
Elan the Bard
Haley Starshine

#993a
Belkar Bitterleaf
Durkon Allotrope Thundershield
"Durkon"*
Roy Greenhilt
Veldrina
Wrecan

#1052a
Elan the Bard
Roy Greenhilt
(I think the 3 frost giants are unique to this page)

#1169a
Belkar Bitterleaf
Blackwing
Elan the Bard
Giant Death Worm
Haley Starshine
Roy Greenhilt
Vaarsuvius

#1170c
CotS Usher with Sandy Hair
Janna
Minrah Elle Shaleshoe
Priestess of Hel with short hair
(2 dominated Council of Elder Guards can be seen in the background of the first panel)

#1182a
Elan the Bard
Haley Starshine
Logann
Logann's Father
Logann's Sister
Quippie
Thirden

#1185a
Durkon Allotrope Thundershield
Haley Starshine
Minrah Elle Shaleshoe