Hey,
I actually was recently sent a link to [url=https
://www.youtube.com/watch?v=7CQZRtTtUSA&feature=youtu.be]this video by Martin Hüdepohl[/url] 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.