You will find "clue bottles" hidden throughout several levels, ranging anywhere in values of 20 to 40. In order to gain a page of the Thievius Raccoonus locked away in a vault near the end of the level, you have to collect ll of the clue bottles for that level. Thankfully, they save the progress often, so if you happen to die between checkpoints, you won't have to re-collect them. The earliest you will get this is on Sir Raleigh's introduction level, which is a rather straightforward level from start to finish.