Skill Tree Pages
When you press the skill tree open key(default 'K'), you can see skill tree pages that contain skills and their connections. Skills can be unlocked in the order they appear in the skill tree.
You can switch skill tree pages by pressing page buttons; Some pages are locked, and you have to meet some conditions to unlock them.
Ability Points
There is a new resource that is required to unlock skills, the **Ability Points**. Each skill has a specific required number of ability points. You can convert experience to Ability points in Skill tree screen.
Pressing the experience orb button will convert exp points into an Ability point. The numbers next to exp orb indicate 'current exp points / exp consumption to convert'.
Ability Stone

You can acquire Ability stones by killing hostile mobs or looting them from chests, like the Skill Books in the original system. By consuming the Ability Stone, you can get an Ability Point without costing experience.
What happened to the Skill books then?
Epic Fight: Skill Tree completely replaces Skill Books in loot tables with Ability Stones. You can not acquire Skill Books in Survivor mode anymore, but still available in the creative tab. However, they don't affect Skill Trees at all, so you have to unlock the skills that are learned by Skill Books with Ability Points.