Sadly I would not recommed Monroe's books at all. They are full of misinformatoin about both the ancient Druids and modern Druidry.
As an example of the nonsense in the "21 Lessons of Merlyn", Monroe states that a female Druid is called a Dryad. Nothing can be further from the truth. A "Dryad" is a Greek tree spirit. A Druid is a Druid, whether they be male or female.
Further, even his choice of spelling is incorrect. In Welsh the word Merlyn means "pony". The correct Welsh name for the mage from the Tales of King Arthur is "Myrddin", meaning "Sea Fortress".
He then goes on to say that the ancient Druids were all vegetarians, a fact which is clearly debunked if one takes the time to read the tales of feasting in the Welsh and Irish myth cycles.
And his books go on and on spouting similar nonsense.
Thank you, Lark, after reading some more about him on the net I will also try to steer clear of his books. His works always looked somewhat dubious to me, that's why I suggested those books very carefully.