I have no idea of the "expert opinion" but I am a huge fan of bilingual books... if both languages are written well/correctly. Sometimes you come across horrid translations, where the words are misspelled and grammar is wrong and nonsensical.
If you get well made bilingual books, you get two for one. I like them since I can read my child the same beloved story in more than one language. You can read it in the language they understand best first, and then read it in the other language and they will understand what the words are meaning as they already understand the story/plot etc. Sometimes I pretend to do this by "reading" an english book in French to my son (translating it myself as I read aloud) as I am bilingual English/French. Though a good french writer would have better vocab than myself, my child still gets exposure to French, and is more open to it if we are using his fave books. And it is less boring yourself than reading the same book over and over!
In chinese, I am learning as well as my son, so I like bilingual books as I can read the book without halting in English, and then stumble through the chinese, and having the english translation there helps me know what I am saying if I forget a word I don't know that well. I just bought three more toddler flap books bilingual chinese/english on abebooks.com
I try to find dvds that are bilingual or multilingual too... my son can watch Annie or Beethoven in English one day and French the next, and he gets twice the language exposure, and doesn't get "lost" in his 2nd language version as he already knows the story and what the people are talking about.
For books, I recommend Mantra publishing and Milet Publishing.
www.milet.com and
http://www.mantralingua.com/home.php
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