Lost Judgment - Skill Books Guide

Skills can be found under your menu, and under Skills in the first row. Some are unlocked during story, some during side cases, and some can be unlocked by finding certain skill books. Most of them are unlocked without any requirements (and most of them requires prior level of some skill tree to be learned, after which they unlock). To unlock a skill, you need SP, which can be obtained by winning fights, finishing missions etc. 

The list of below skills is made based on the order in the menu, so it's easier for your to check what you still miss.

0:01 - #01 Book of the Wounded
0:39 - #02 Book of the Ardent
1:27 - #03 Book of the Ravenous
1:41 - #04 Book of the Brink
2:20 - #05 Flux Fissure Redux
2:59 - #06 To Crush a Dragon
3:27 - #07 Skateboard Rocketeer
3:48 - #08 Memoirs of an Eagle
4:03 - #09 Excerpts of a True Crane
6:51 - #10 White Snake Analects
7:21 - #11 Book of Slithering Snake Vol. 1
7:46 - #12 Book of Slithering Snake Vol. 2
8:32 - #13 Book of Slithering Snake Vol. 3
8:57 - #14 Showa Duel Records: Man
9:22 - #15 Showa Duel Records: Earth
9:37 - #16 Showa Duel Records: Water
10:25 - #17 Showa Duel Records: Fire
10:50 - #18 Showa Duel Records: Wind
11:42 - #19 Showa Duel Records: Sky
12:02 - #20 Superstitious Eater’s Guide
12:22 - #21 Surprise Backhug
12:43 - #22 Ramblings of a Hermit
13:09 - #23 The Walkin’, Talkin’ Hermit
13:37 - #24 The Loyal Dog of Bushu
14:09 - #25 Unofficial VR Strategy Guide 2021
14:32 - #26 Unofficial VR Strategy Guide 2021 [Append]
15:07 - #27 Unofficial VR Strategy Guide 2021 [True]
15:47 - #28 The Art of Gluttony

Skill Book List (Detailed)

• Book of the Wounded - Obtained during the story in Chapter 2. You can also purchase it from Onodera’s Wares for 3,500 SP, but I don't see a reason why.
• Book of the Ardent - Finish Squirrel requirements on the brick pillar just outside Seiryo High School (entrance to the yard).
• Book of the Ravenous - Reward for completing Side Case "The Forbidden Taste".
• Book of the Brink - Reach max level Pawpularity with the Orange cat located by the Homeless Camp in Ijincho.
• Flux Fissure Redux - Finish Squirrel requirements inside Welcome Pharmacy in Ijincho.
• To Crush a Dragon - Obtained during the story in Chapter 12. You can also purchase it from Xi Ying Tang in Ijincho for 78,000 Yen if you prefer.
• Skateboard Rocketeer - Can be purchased from Skateboard Point Exchange shop for 35,000 pts.
• Memoirs of an Eagle - Reward for completing Side Case"My First Errand".
• Excerpts of a True Crane - Beat "Gauntlet: Air Yagami" mission. You access Gauglet missions from the game starting screen. This one requires you to beat enemies with air kicks (only air kicks will do the damage), so use your run to the walls + kick moves, or L1 + X and then kicks to perform air moves. You have a lot of time to do this, so it should be fairly easy.
• White Snake Analects - Finish Squirrel requirements inside the Dinosaur’s mouth outside of Wette Kitchen in Ijincho.
• Book of Slithering Snake Vol. 1 - Can be purchased from Ebisu Pawn Kamurocho for 30,000 Yen.
• Book of Slithering Snake Vol. 2 - Finish Squirrel requirements at the top of the stairs just south of Ichiban Confections.
• Book of Slithering Snake Vol. 3 - Can be purchased from Onodera’s Wares for 10,000 SP.
• Showa Duel Records: Man - Batting Center Prize Exchange for 2,000 pts.
• Showa Duel Records: Earth - Reward for completing Side Case "The Chaos of the Masked Writer".
• Showa Duel Records: Water - Finish Squirrel requirements on the southern fence just next to the Hello Work taxi stop.
• Showa Duel Records: Fire - Reach max level Pawpularity with the tiger cat located by the Homeless Camp in Ijincho.
• Showa Duel Records: Wind - Finish Squirrel requirements inside the Men’s Entertainment Box building in Kamurocho.
• Showa Duel Records: Sky - Can be purchased from Onodera’s Wares for 3,500 SP.
• Superstitious Eater’s Guide - Reward for completing Side Case "Squirrels Just Wanna Have Fun".
• Surprise Backhug - Can be purchased from Coming Boon for 3,500 Yen in Ijincho.
• Ramblings of a Hermit - Can be purchased from Outdoor Shogi Exchange shop for 250 shogi points in Ijincho.
• The Walkin’, Talkin’ Hermit - Finish Squirrel requirements just to the opposite of Smile Burger in Kamuorcho.
• The Loyal Dog of Bushu - Can be found by searching with Ranpo the Detective Dog at the south end of Isobe Lane in Ijincho. 
• Unofficial VR Strategy Guide 2021 - Can be purchased from Onodera’s Wares for 100 SP.
• Unofficial VR Strategy Guide 2021 [Append] - Can be found by searching with Ranpo the Detective Dog by the Hills Garden in Kamurocho (check around the map label, Ranpo will show you the way).
• Unofficial VR Strategy Guide 2021 [True] - Finish Squirrel requirements on the white sign in the dead-end alley just to the east of Sushi Zanmai in Kamurocho.
• The Art of Gluttony - Can be purchased from Onodera’s Wares for 50,000 SP.