You can use them at any level. But good thing you haven't used them yet, as it's extremely unlikely there are any champions you own that would be worth booking at the moment.

You should only use rare books on your starter champion for now. Don't book anything else, and especially don't use epic or legendary books on anything yet.

Feel free to post a screenshot with your champs and we can recommend which, if any, you should be booking.

each type of book should be used on the same rarity champion: 

Rare books can ONLY (and should) be used on rare champions

Epic books can be used on rare and EPIC champions (you shouldn't use them on rares)

Legendary books can be used on any kind of champion, but they're very hard to find, so ONLY use them on Legendary champions

Thanks for the advice. I love reading books and writing short essays about them. It helps me to better remember the essence of the book, the main characters. Now I am writing a term paper and the company https://writix.co.uk/pay-for-coursework helps me, so there is not much time to  read books for my pleasure.

 Also if you join a clan and do the clan quests to earn clan gold you can go into the clan shop and buy clan books! errr... I mean rare books! lol But yeah, there's basically a slot in the clan shop that has a 1:3 chance of having up to 5 rare books each week which can be a great opportunity when you're starting out to help book your Frozen Banshee, Apothecary, etc. You can also get some really good accessories in there but I would prioritize the books. If you have any friends who you can get to create accounts and play at least till level 40 you can pick up two epic books per friend from the referral program which is well worth it, you may even get some shards out of the deal.

locked for necro'd spam reply.