There is nothing wrong with study. However, looking up *basic card meanings* isn't like a doctor looking up obscure drug interactions (though you wouldn't want an ER doctor to have to do that for basic drug interactions, now would you?). It's like an ER doctor not knowing how to put in a central line.
Basic card meanings are an essential foundation. There's nothing wrong with continuing study and reading other points of view on card meanings in your own time, but if you see a card and don't immediately have a sense of its meaning (and a complete sense of that card, other potential meanings, and potential significant card combinations that include that card), then you should not be reading professionally as a Tarot reader. (Of course, there are some readers who are primarily 'psychic' and just use cards to jog their intuition... I won't speak to that type of reading, as that's a different ball of wax and can have no strict standards except past and present accuracy, since there's no formal standard to personal intuition.)
Sure, I believe you learn forever, but there should be some standards before hanging up a shingle. In the analogy, doctors have intense schooling and certifications to go through; Tarot readers do not, so we have to be vigilant in holding ourselves to high standards.
If you still don't have an extremely strong sense of each of the 78 cards (and some sense of the combinations between them), then it's not time to read for the general public. Free readings on a site such as this, where it's transparent what's happening, are good. Readings for friends and family are good. Readings for friends of friends (in college, I used to read at parties for people who thought it was a joke, for free of course, and that was GREAT practice on halfway drunk people who had no expectation!) is good.
Reading for strangers who are often in emotional situations, who need a counselor and might take your advice, whatever the legal disclaimer says, is a huge responsibility that is still out of one's depth at that stage.
P.S. I will also say that if you cannot read in person, you cannot ethically read professionally. Just because an email reading *can* be open book doesn't make it right, unless your clients know that you may be consulting a book. I have a feeling most sitters would not want their Tarot reader to consult a book, so this seems kind of sneaky to me.
Reading in person (for non-paying clients, who realize where you are with your craft---kind of like student hairdressers, I suppose) is also the best way to learn to actually read the cards!
As far as how to learn them all, there are any number of ways. Of course, books are good if you truly don't know the cards. What is best for you is really dependent upon where you are in the learning process. Books, knowledge, and study is excellent but only takes one so far; then, it becomes time to forge a personal connection with the cards. But if you're seeing spreads and coming up with no immediate answers, it's not yet time to read professionally.