The construction Ho letto un davvero buon libro is ungrammatical. If the adjective is modified by an adverb, it must go after the noun. For instance, Ho letto un libro davvero bello.
But un libro buono is not the same as un buon libro; consider un uomo povero and un pover uomo: they have different meanings. That's a general rule, the adjective preceding the noun gives the construction an “abstract” (better, perhaps, figured) sense. The concrete sense can't be applied to a book.
But "un libro davvero buono" (a really good book) can be used, because the adverb modifies the adjective giving it the figured sense. The construction Ho letto un libro davvero buono, however, sounds a bit formal, so another construction is possible
Ho letto davvero un buon libro
which means the same as Ho letto un libro davvero buono, unless, in speech, one gives special emphasis to the adverb. This could be rendered, in writing, with commas: Ho letto, davvero, un buon libro (in English this would become Really, I read a good book).