Your Legend Grows is a class quest, like the ones you got at level 15 and 30. At 39 all classes receive a mail with an attachment to start the quest Your Legend Grows. The quest asks you to talk to your class trainer in Bree, which you do.
So far so good, but now the hard part begins: understanding how to progress the quest. Or, simply realizing there's more quest left to do! The weird implementation of this quest means that the next steps are easily-missed and confusing. I'll walk you through, but first…
Why should you care?
Good rewards! This quest branches into three deeds, each of which rewards a class trait point and 10 LP. Three trait points is a big deal!
Also, these deeds involve killing mobs. It's well worth progressing the deeds to Step 4 (below) ASAP since your normal leveling usually kills the same mobs. Otherwise, prepare for a grind if you come back later to do these.
1 - Talk to your class trainer to complete the Your Legend Grows quest
2 - Buy the Legendary books from your trainer. In the fine print of the Grow Your Legend quest, the trainer instructs you to buy three Legendary books from him. These cost about 190s each. As you buy each one, it automatically starts a class deed with the same name as the book.
3 - Talk to someone who can tell you more. These are technically deeds, but they work more like a quest. First objective: talk to the person noted in the deed.
For example, Hunters talk to Legolas in Rivendell:
After your conversation you'll note that the objectives for all three deeds have changed.
4 - Collect 8 missing pages for each book.
Each deed now lists 8 pages which must be collected. This is the real task of the deed.
Pages for all three book are an uncommon drop in mid-40s to low-50s level areas: Misty Mountains, Angmar, Eregion, and the early parts of Moria. Any human or orc-kind enemy can drop them, including normal mobs.
Collect all the pages to finish the deeds and get the rewards.
If you completed the quest earlier but didn't buy the books, don't fret. Simply buy the books from any trainer for your class to start the deeds.
Hope this helps!