There are many types of souls in LOTB, so it might be hard to differentiate them all.
Some souls come as a whole, and others come from fragments. The fragmented ones allow you to see the available character list in the souls, so don't hesitate to go to your fragments and look up what kind of characters you can get from a particular soul.
The Book of souls interface also has an information button (that circled I in the top right corner). Click it to open an information screen where you'll find exact (hopefully) drop chances for each toon class in the soul (ie : 85% 3*, 14% 4*, 1% 5*).
You'll also notice as you play longer and longer that those souls tend to accumulate as soon as you no longer "need" to open them, leading to stacks of souls cluttering your BOS. Don't let yourself get drowned in them, because at some point the game will prevent you from doing quest because of "inventory full". Cleaning them up might seem cumbersome but you'll soon realize that with the new fragment system, cleaning up commons gives you rares, etc. So do your cleanup when you need to.
Opening souls isn't cheap, it costs gold and some souls type require a lot of gold (epics for example). There are times when you know you'll be opening lots of souls (sacrifice events for example). Farming for gold at that moment can be a drag, so plan accordingly.
On the topic of gold, it is good to keep in mind that fragments can be converted to precious souls fragments. Meaning your leftover unused fragments become a valuable resource. There are also times when you'll have to make a choice on keeping some fragments or trading them for precious. Notice that all souls fragments do not convert to the same amount of precious fragments. The really rare souls give a 1:1 conversion rate (mythicals), while more common ones can get up to 5:1.
Precious souls are a special type of soul that can contain hearts or idols, with the 1*, 3* or 5* quality..
Hearts are sacrifice resources, these characters give out good essence value, with 1* scoring 600 essence, 3* at 1049 and 5* at 3200 essence respectively.
Idols are gold resources, they can be resold for decent gold, with 1* giving out 50K, 3* at 300K, and a whooping 1 million gold for 5* idols.
Levelling hearts for sacrifice can increase their value up to 1000/1500/4000 essence for the 1/3/5* hearts. evolving them is a waste of ressources.
Levelling idols gives minimal value increase, so don't bother.
Some of the rarer souls in the game give out 100 frontier coins when opened, making them the prime source for frontier coins, these include the following :
It's important to note that most souls give out fragments of other souls, building a chain of better and better rarity culminating in souls containing exclusively 5* toons :
-Common souls drop rare soul fragments, rare souls drop epic soul fragments, epic souls drop mythical soul fragments
-Common class souls don't drop rare class soul fragments (class souls are warrior, sentinel, magus, gunner or assassin or gunner/assassin souls)
-Rare class souls drop epic class soul fragments, epic class souls drop mythical class soul fragments
-Heroic souls don't drop legendary or epic heroic souls fragments, they drop fragments of a designated 5* toon for the rotation
-Legendary souls don't drop fragments at all, except for...
-Legendary Heroic souls drop Mythical Heroic Soul fragments
The ratios vary greatly, so opening 100 common souls might give out enough fragments for one rare souls, while opening 10 rare souls usually drop enough for a full epic soul.
Be smart, count those, know how many fragments you need for you next epic (4*) or mythical (5*) soul.