Well no book but a long time gamer here:
Why not take a look how other games made it. Most MMO:s have forums. Post in their general board and ask the players themselves what make them stay or what features they want.
My pro tips are:
Implement a guild system and a solid friend system in the game. Encourage players to play together but don't force them. Some guildsystem where you not only earn exp but also get guildexp and then guilds can buy benefits for their membera (maybe not more HP or Damage since it will be unfair for guildless people but some discounts in shops ingame or some special looking pet or mount or banners/clothing).. Maybe if you have some area where only guildmembers can enter say some battle arena..then you can give HP and dmg buffs inside that area since non guildmembers cant access there anyway.
Also hire a comunity manager that let the community in on some parts of the development. Listen to the players input but do not blindly do what they ask or you will have one big fat nerf fest if so.. But if 90% of the players say something is not fun you probably should take a look at it. Give reasons to the players why you decide to not do something they request instead of just pretend it is raining. If they are asking on a nerf on barbarians but you do not feel it should be so then just tell them we don't want to do that because of this and this. I think communication and honesty is very important if you want to keep the player base.
Have some endcontent in place from the start. Figure out how the endgame shall work BEFORE release. I think this is a mistake many games made.. They thought it will take 6 months before players get there so we have plenty of time to implement it.. Then they have some guild of hardcore players that reach endgame after a week and then spend 2 months on the forums asking for endgame and then 2 months when they are pissed off and wreck havoc on the forums and badmouthing the game and then players and new players get the feeling there is no endgame or point in leveling up.. Few games manage to re attract players after the player uninstalled the game even if comments turn positive later on.
About fun: We are all wired differently. Myself I love character progression and loot, I don't care about story at all just gameplay and see my char become stronger. My friend is the complete opposide. He just want to progress in the story and read every line in quest descriptions..
But get the mechanics done first so gameplay is smooth and fun and then focus on story or progression because if the gameplay/fighting/whatever is not intuitive and good no story or cool loot will help the game.
Just my 2 cents ?