I'm looking for a book, or any other source, which lists words that are morphologically related, like this:
imagine verb imagination noun imaginative adjective
medic noun medicine noun medical adjective medicinal adjective
And also this kind of relationship:
head noun ahead adverb, adjective
Dictionaries will give you only partial information, and in a very inconvenient way. For example, if you look at the definition of imagination, the words imagine and imaginative are not mentioned there, nor any reference to them. I've checked several dictionaries, and you may be lucky with other ones, but I have not found one which lists all the relevant information in a straightforward manner.
Etymology dictionary is also not suitable for this task. It doesn't give you all the relevant forms of the word, and it does give historical information which is not relevant here.