Understanding this usage of を

こんにちは (Konnichiwa) イ リ ニ,

Thanks for your question!

を (O) is the object particle used to identify the direct object (the noun that receives the verb directly) of the sentence. In other words, を (o) tells us what's happening to the object. 

In this sentence, the object is the "book" 本 (hon), and it is being "put in" 入れる (ireru) the bag. We can show the connection between 本 (hon) and 入れる (ireru) using the  particle を (o).

Rest assured, this is the same use of を (o) that you learned in lesson 2.6; however, this sentence might be a little confusing, since 本 (hon) and 入れる (ireru) aren't right beside each other. 
It might be easier to understand if we re-write the sentence like this:

鞄に本を入れます。 (Kaban ni hon o iremasu) "(I will) put the book in the bag."

As you can see, this still has the exact same meaning! 
Sentence order isn't very important in Japanese (as long as the verb comes last).
As long as you use particles correctly, you can show which words are connected, even if they aren't right beside each other. 

In the case of this example, by attaching を (o) to 本 (hon) "book," we know that this is the object receiving the verb 入れる (ireru) "to put in," regardless of where it is placed in the sentence. 

Your sentence is just about perfect, you just need to swap the が (ga) after 本 (hon) for を (o). It should look like this: 

私は本を鞄に入れます。(Watashi wa hon o kaban ni iremasu.) "(I will) put the book in the bag."

Or, you could also say:

私は鞄に本を入れます。(Watashi wa kaban ni hon o iremasu.) "(I will) put the book in the bag."

I hope this helped! Please let me know if you have any more questions!

勉強を頑張ってください！ (Benkyō o ganbatte kudasai!)