Ryu Books are key items used in the .hack saga which can track certain accomplishments fulfilled by a player, giving them certain bonuses such as movie clips or desktop music.
Ryu Books have an special category in the Key Items menu. There are a total of 8 books; each one that can be unlocked at certain times when using Data Drain skill. Each book is added to the Key Items, and records a specific data like enemies defeated, or number of trades made. When you reach a certain number, the Ryu Book will generate a Picture, Movie or BGM in the ALTIMIT screen.
- Ryu Book I: It shows the number of areas visited and total play time.
- Ryu Book II: It shows the amount of Magic Portals activated in Fields and Dungeons.
- Ryu Book III: It shows the amount of players you have met and trades.
- Ryu Book IV: It shows the amount of monsters you have fought.
- Ryu Book V: It shows the amount of GP in gifts to other players.
- Ryu Book VI: It shows the amount of Gott Statues, treasures, boxes, casks, jars, and bones cleared.
- Ryu Book VII: It shows the amount of Spring of Myst and Symbols activated.
- Ryu Book VIII: It shows the amount of Grunty Food and the Gruntys raised.
It is uncertain that Shugo's bracelet is capable of this. It seems likely they were added for gameplay purposes, and aren't 100% canonical, though their relationship to the bracelet is shown in Analysis.