I think the rule as intended is that you are familiar with the creature. I don't see any rules in the PHB about gaining additional shapes, so it comes down to the DM. (opinion)I would probably be okay with drawings with a nature check of some kind? Again, lots of ways to handle it.
Beside the standard "up to the dm" answer, let me link you an Unearthed Arcana PDF. On the last few pages, it provides more of a framework for those who want more concrete rules (Keep in mind this is playtest material). They suggest:
Observation. You learn the beast's shape after observing its behavior for at least 1 hour and succeeding on an Intelligence (Nature) check with a DC equal to 10 + the beast's challenge rating. For this observation period, your vantage point-whether physical or magical-must be within 150 feet of the beast. If you previously spent at least 1 hour reading a scholarly work about the creature, you have advantage on the check.
Interaction. You learn the beast's shape after interacting with it peacefully for 10 minutes and succeeding on a Wisdom (Animal Handling) check with a DC equal to 10 + the beast's challenge rating. For this interaction period, you must be within 15 feet of the beast, and if you spend at least a minute petting it, you have advantage on the check.
I am currently play testing that (as the DM) and love it.
Comment asks me to elaborate on my (opinion) above and the fact that I am using the UA.
Before the UA came out, we ran a homebrew game. A druid player was a bit upset that most of our campaign was urban / ghost hunting. They also felt uncomfortable asking to take the story from the group so they could watch bugs. Of course, I encouraged them to take the reigns, but also offered them the ability to study writings/drawings of the creature. Based on the level of knowledge they had, I'd set a DC (usually 15 or so) for a nature check). The role determined if he gained that beast shape or not. I admit this is a bit more "wizard"-y than a druid, but you could possibly consider combining the systems... give advantage on the check if they have a decent understanding of the creature.
At any rate, I would go with the UA article at this point. If the player wants to be some sort of nature documenter or something, I may tweak the system to make sure they have fun.