Rainer Maria Rilke seems to have been receiving letters from unknown correspondents often, and he answered them with remarkable seriousness and compassion; the result is a separate body of epistolar works of poetic prose (apparently, an infinite well of quotes for self-help books up to this day). It always makes sense to go back to the source, especially if it's that beautiful (even in translation). And ten letters, albeit not very short ones (after all, they aren't e-mails, right?), don't take very long to read.
Even though it's prose, it's written by a great poet, with density and intensity of expression closer to poetry than to prose. So the idea of summarizing the content of the book seems somewhat preposterous (actually, not just seems, it is preposterous). Instead, I want to focus on one particular thought, which appears in the very first letter (which is the answer to the young poet's first missive, in which he sent Rilke some of his own efforts for critique). Quite a common situation, played out a gazillion of times both in real life and in literature: a young artist addresses an older one with the ultimate question, "Am I a poet? Should I be doing this? Am I any good?"
Even though I strongly empathize with this answer (as there is little else one could possibly say), I also have some qualms here. For one thing, this obviously presupposes the existence of some other you, different from the you who asks; this other you knows the answer, you just have to listen to it. This assumption is, of course, supported by a variety of psychological (and para-psychological) theories, which give various names and characterizations to this (often dysfunctional) family of you-s, some of whom know answers that others don't. And still, this other you, which (as Rilke's advice suggests) is outside his correspondent's regular conscious awareness, and which holds the ultimate answers, -- this other you might happen be filled with all kinds of strange and contradictory beliefs, deeply internalized voices of others, and whatnot. I am far from playing a psychologist here, but our unconscious, especially in youth, is quite likely to be filled with lots of unfiltered, outdated nonsense, at least at some level. Why should one take its "answer" as the ultimate truth?
My other doubt has to do with the exact formulation of the question Rilke suggests. The question is whether one must write, whether one would actually die if they were denied writing. If not, if its not an absolute, basic, unquestionable need, it's enough to drop it and never write again. This seems to be in perfect contrast to the current wisdom we are showered with daily nowadays: that one should "create" (in whatever form) if only one can and wants to (or even if one doesn't necessarily want to): that's good for you, to nourish your creativity, right? You don't have to be incapable of living without it; you just have to find your "motivation". It's better than watching TV, or shopping, or eating ice-cream anyway, isn't it? Except, of course, there is also this third, ever-present, option: looking at a Rembrandt, or reading Rilke, or listening to Mozart...
To tell you the truth, this is where both my doubts converge back into one, because my own "I"s (whatever they are), neither of them can tell me whether either of these two contrasting approaches is the right one. I deeply dislike both of them, albeit for different reasons. Maybe, in the end, we must accept that the system of modal verbs (like "must" or "want") of whatever language one was born into is just not enough for this particular purpose. So I leave this question without a verbal answer; instead, here is a painting of mine, which represents my true answer: it translates a poem by Alexander Blok, which, once upon a time, used to be my first real window into the nature of artistic endeavor.
I don't give links to the book itself here, because I chose a translation randomly, and I am not sure which one is better (if any); so if you decide to read the book (which I strongly recommend, especially if you are a young poet, or painter, or musician; or even not so very young, come to that), choose one yourself at your favorite place: there are lots of both e-editions and traditional editions available.