In the book Rick says this bus is one ticket per direction. We're staying just off Rue Cler and want to take the bus east. In the book it looks like the bus ends and then heads east. My question is, if I get on near Rue Cler, will I have to pay another ticket to go east?
If I understand your question, you are asking if you get on the No. 69 before it reaches its western terminus (Champs de Mars), will you have to pay another fare to begin the eastern route. The answer is yes. All passengers are required to disembark at Champs de Mars, then the bus goes to the other side of the Champs where passengers get on to begin the western route.
You can catch it one block farther down at rue St. Dominique to go towards St. Germain, etc. you don't have to go all the way to Champs de Mars to catch it. Just go down rue Cler, cross rue de Grenelle go to rue St. Dominique and turn right and walk a few steps to the bus stop. These are one way streets, that's why the bus comes one way and goes the other. Like the book says, totally worthwhile for the scenery.