"The book that is not famous is under the table."
Translation:Az a könyv van az asztal alatt, amelyik nem híres.
From what I have gathered, ami is used to refer to entire sentences ("He came late again, which was bed.") or to things that are not expressed by an explicit noun ("I eat what moves"), while amelyik is used for explicit nouns ("I eat the cake which your sister made").
English and Hungarian do this kind of thing differently.
In English, the relative clause that is not famous comes directly after the noun phrase the book, and being that closer, tells you which book it is.
In Hungarian, the relative clause amelyik nem híres is a bit further away, and so to signal that you are going to specify which book you mean, you use az a könyv, which is not necessary in English.
So you can't translate this sentence a word at a time -- you have to translate the entire sentence, transforming the grammar from English into Hungarian.
Találtam ehy kitűnő cikket Nádasdy Ádámtól: http://seas3.elte.hu/delg/publications/modern_talking/14.html