Neither sentence demands that you use one or the other tense.

I've been waiting ages to read the book.

I've waited ages to read the book.

A native speaker might be more likely to use the first if the wait is still not over yet, and the second when the wait is over, either because the book has finally become available or because the person waiting has given up waiting. But that is just a tendency, not an absolute distinction between the two.

• I've spent ages preparing for the party.

• I've been waiting ages to read the book.

• I've been spending ages preparing for the book party every week.

You could use the PP continuous but it would imply a repeat activity, for example. It requires making changes to the meaning of the sentence, as I have done, in order to show that it's more than one thing.

The simple present and present continuous function similarly to the PP and PP continuous. Compare the grouped sentences.

1) - I spend ages preparing for our party every week. [general statement]

2) - I'm spending ages preparing for the parties every week. [ongoing at present] [1) simple present; 2) present continuous]

3) - I've spent ages preparing for our party this week. [started at some unspecified time in the past and is true as time of speaking]

4) - I've been spending ages preparing for the last three parties. [started in the past and continues now at time of speaking as an ongoing thing but must work as a general statement] [3) present perfect; 4)

"I've been spending" would be used for something that happens more than once, started in the past and that is true at the time of speaking.

"I have spent ages" and "I spent ages": the first merely tells us you have spent ages on it up to the time you say this; The second one means you did so at some specific past time that is now over.