First of all, and very importantly, "poética" doesn't refer to poetry here. It means the nature of the author's literary discourse, the underlying rules of his literary art. Its meaning is expressed in the fourth DRAE definition of "poética":

"4. f. Conjunto de principios o de reglas, explícitos o no, que observan un género literario o artístico, una escuela o un autor."

In English, it is called "poetics". This use of the word goes back to Aristotle's literary treatise Poetics, basically about dramatic art. Dostoyevsky may have written some poetry, though he is not known for it (in his first novel we find this memorable quotation: "As for poetry, I may say that I consider it unbecoming for a man of my years to devote his facilities to the making of verses. Poetry is rubbish. Even boys at school ought to be whipped for writing it", though this may not reflect D's own view); this passage must be referring to his novels. There is a famous book by Mikhail Bakhtin called Problems of Dostoevsky's Poetics.

I am sure "elusive" is the best word for "escurridiza": difficult to grasp, difficult to pin down, not obvious.

The difficult word here is "forzada". It literally says that D's poetics were "always forced to be elusive". This doesn't really work well as a translation, I feel. I think it must mean "forced by the author", and therefore I think this can be interpreted as "deliberately". I don't think it means "inevitably", because "forzada" implies that something has been done to make the author's poetics elusive.

By using an adverb for "forzada a ser", I think we can follow the syntax of the Spanish; the pre-posed adjectival phrase is still a bit cumbersome, but I think acceptable: "this author's always deliberately elusive poetics". Alternatively, you could say "this author's poetics, which are always deliberately elusive", but I find that produces a more awkward sentence structure, and there's also the constant problem of whether "poetics" is a singular or a plural noun (practice varies on this).

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Note added at 2 hrs (2013-03-20 10:12:11 GMT)
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Then again, perhaps the author did mean "inevitably"; I'm not entirely sure.

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Note added at 4 hrs (2013-03-20 12:17:07 GMT)
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Or perhaps the most accurate rendering for "forzada a ser" would be "perforce": "this author's always perforce elusive poetics". This is not such a strange expression in literary critical discourse.

"historical knowledge is always perforce conjectural"
http://books.google.es/books?id=UPzEu9uqazQC&pg=PA52&lpg=PA5...

Though if you choose this it would probably be better to rearrange the phrase and say something like "this author's poetics, which is/are, perforce, always elusive".