Ratgeber is another word like "Reiseführer": It can be a book or a person.
As a book it is meant to help you with things, for instance tax declarations, repairs, keeping chicken, you name it. Also with travelling! So a Reiseführer can also be called a Ratgeber, one that gives advice ("gibt Rat") on travelling.
As a person a "Ratgeber" is something historical - "die Ratgeber der Königin" - nowadays the word has been replaced by "Berater" and anglicisms like "consultant"
Certain verbs in German - and English too technically - are followed by Predicate Complements; "sein" being one of these. Basically it means that the object that complements the verb doesn't decline, normally because they're the same idea e.g: "Er ist und bleibt ein Idiot" - He and the idiot refer to the same person "Der Song ist der Hit des Jahres" "Er bleibt mein Fruend"
This site isn't bad for explaining it: http://www.vistawide.com/german/grammar/german_cases_nominative.htm