Hey,
I actually was recently sent a link to
this video by Martin Hüdepohl on the guidelines for the fair use of LEGO in publishing. He's not from LEGO legal, but he is the author of some AFOL-driven books. Watch the video, and access the documents form TLG that he mentions, but it essentially comes down to these main points:
-You can't use the LEGO logo and some other images unless you are licensed by TLG (e.g. DK has a business relationship with TLG).
-There are restrictions on the proper way to use the word LEGO or other trademarked terms (e.g. Duplo, Ninjago, etc). Things like all-caps, never say "Legos" etc. Sometimes (but not all?) you're supposed to use the little registered trademark symbol.
-There is some restriction on how the word LEGO is used in the title.
-There is a specified disclaimer saying that this work is not supported or endorsed by TLG.
Aside from those things, it seems that you are free to have a book with LEGO illustrations, write a book with instructions to build your own custom models, write a reference book about LEGO, etc.
BTW, if you are going to publish such a book, I would try to do it through a publisher that has a proven AFOL track record - i.e. NoStarch or Skyhorse (there are some others based in Europe as well). They would both already know and be able to advise you on legal mumbo-jumbo, and also they have a record of marketing to the LEGO market.