The Ryu Books are key items that you receive when data-draining in the
.hack// series. You will get one for every ten data drains, up to Ryu Book VII.
The Ryu Books track various statistics, and will reward you for hitting specific numbers, ie, for meeting Grandpa five times. You can get all of them in the fourth game, Quarantine. Otherwise, a few more rewards unlock each game, as listed below.
The rewards consist of images and music with which to decorate your desktop, and videos of cutscenes, which you can view after gaining the data flag, ie, finishing each installment. The rewards always unlock in the same order, no matter which achievements you get.
I was originally going to list all the achievements you can unlock, and which game you can first get them in. But, really, that's boring and it's been done before. I'm only listing the highest numbers you need to get, so you know what to aim for. You'll only unlock the final reward in Quarantine, but you won't miss any if you hit these numbers then.
Ryu Book I
Chronicles Areas and Playtime
- 210 Areas Visited
- 60 Hours Playtime.
If you struggle with this, just hop in and out of a few areas, then leave the PS2 on overnight. But, really, you shouldn't struggle.
Ryu Book II
Chronicles Magic Portals
- 1700 Magic Portals Opened.
- xx Field Portals Opened.
- 75 Dungeon Portals Opened.
If you struggle with field portals, just go to a low level area and open a few. For field portals, Lambda Lonely Ghostly Feeling. Three. You're welcome. For dungeon portals, try Theta Chosen Corrupted Spiral, which has six.
Ryu Book III
Chronicles Players You Have Met
- All Players Met.
- xx Trades Made.
If you just talk to a handful of people as you run around the root towns, you should meet most of them without trying too hard. Some characters won't appear until the game has been completed.
Ryu Book IV
Chronicles Monsters
There is an excellent bestiary here. When you reach Quarantine, simply look up any monsters you've yet to meet and go exploring. Don't bother doing it earlier; you'll probably run into them anyway, and the effort will be redundant.
Ryu Book V
Chronicles About Your Friends
I hit twice that just by making sure everyone had the best weapons in each game.
Ryu Book VI
Chronicles Treasures
- 1500 Treasure Boxes Opened.
- 1400 boxes, cask, jars and bones destroyed.
- 85 Gott Statue Treasure Opened.
If you leave and re-enter a room, boxes and casks will reappear. Try Omega Chosen Solitary White Devil for Gott Statues; a two floor dungeon, with no forced battles.
Ryu Book VII
Chronicles the Spring of Myst and Symbols
- 55 Encounters with Monsieur.
- 55 Encounters with Grandpa.
- 140 Symbols Activated.
For Symbols, try Omega Bigoted Snowflake's Capsule. There are two immediately to the west of the dungeon entrance. You'll easily meet enough grandpas and monsieurs while completing the item list.
Ryu Book VIII
Chronicles About Grunties
- All Grunty Foods Collected.
- All Grunties Raised.
- 230 Grunty Foods Obtained.
See my post here for where to find the food and the stats for the Grunties. You should pick up enough Grunty food easily if you just grab a few whenever you go past.