There are many different collection books in the game!
Many of them with cool prizes! Some of them are still under construction and will be added in the future!
Color Collection book
Here you can see all colors, that can be found "in the wild" (events, battlefield, pony exchange) in WHV. Collect all horses from the "need to be collected"-section to finish the collection book. When a horse of that color joins your herd the color will disappear from the "need to be collected"-section and move into "already collected" (Leadstallion and leadmare do not count).
Once the "need to be collected"-section is empty you can receive a reward and start the collection anew to get rewards multiple times. Rewards are related to the herdsize (due to differences in difficulty). All colors have the same rarity to be found in the wild and can be found as male or female.
Rewards depending on herdsize:
30 and under: 1 collectible horse and 450 xp
50 and under: 1 collectible horse and 250 xp
100 and under: 10 Token and 100 xp
over 100: 50 xp