der Ratgeber
guide
A book, magazine, or program that gives practical advice on a particular topic.
Ich habe mir einen Ratgeber über die richtige Geldanlage gekauft.
I bought myself a guide about proper investment.

guide / adviser
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guide
A book, magazine, or program that gives practical advice on a particular topic.
Ich habe mir einen Ratgeber über die richtige Geldanlage gekauft.
I bought myself a guide about proper investment.

The word is made up of "Rat" (advice) and the verb "geben" (to give). A "Ratgeber" is therefore literally someone or something that gives advice. This structure helps you understand both meanings: person and book.
Dieser Finanz-Ratgeber gibt viele nützliche Tipps zum Sparen.
This financial guide gives many useful tips on saving money.
You can tell whether a person or a medium, e.g. a book, is meant from the accompanying verbs and adjectives. Verbs like "lesen" (to read) or "kaufen" (to buy) point to a book, while personal descriptions like "ein guter Freund und Ratgeber" (a good friend and adviser) refer to a person.
Ich habe mir einen Ratgeber über Gartenarbeit gekauft.
I bought myself a guide about gardening.
To indicate the topic of a guide medium, German typically uses the prepositions "über" (+ accusative) or "für".
Sie sucht einen Ratgeber über die Börse.
She is looking for a guide about the stock market.
advice / council
'der Rat' and 'der Ratgeber' both have to do with help in making a decision or dealing with a problem. 'Rat' is the recommendation itself: someone tells you what you could do. 'Ratgeber', on the other hand, is the person or book that gives such recommendations. So you can get 'Rat', but read or ask a 'Ratgeber'. The words are easy to confuse because they look very similar. The important point is: 'Rat' is the help as content, while 'Ratgeber' is the source of that help.

Für gesunde Gerichte mit den Kindern kaufen wir am Wochenende einen Ratgeber.
At the weekend, we buy a guide to healthy dishes to make with the children.
Nach dem Gespräch folge ich dem Rat meiner Lehrerin.
After the conversation, I follow my teacher's advice.