Cambridge Word Routes Αγγλικά-Ελληνικά
Θεματικό λεξικό της σύυχρονης αγγλικής γλώσσας
This is published by Cambridge University Press
ISBN 0521 480248 Hardback
ISBN 0521 445698 Paperback
I find this an invaluable reference book.  Covering most topics from wild animals to parts of buildings.  Also covering specific words for example Danger - then giving various explanations and words within this such as pitfall,jeopardy, risky usafe etc etc similar to a thesaurus.  I find if studying a parrticular theme in classes such as hobbies or the environment this book helps tremendously with the relevant terminology.  It can be bought online.  Can't be certain now of the price but I think around £23? for paperback edition.  Well worth it in my view.

Similarly the Compact visual dictionary Greek/English available online from Bibliagora is a good pictorial dictionary, again in topic sections.  I have found this useful in a number of situations.  I couldn't make myself understood in English or Greek to my German friends (I know very few German words but am trying to learn) when trying to explain I liked blueberries.  I looked in the fruit section of this book and was able to show them the relevant picture.  Problem solved!