“读书啦”is typically used in two ways. First,when sb asks:"what are you doing?" you can answer:"读书啦！"that means "I am reading/learning/studying." But in this condition "读书啦" is very girlish, because girls tend to use the word of “啦”，for example,知道啦! 好了啦! Second,when you want to ask or order your friend, little brother\sisiter or child to begin to study, you can call:"come on, let's get reading/studing."that means "it is time to study." It can be used both by male and female.

question :你在干什么？answer: 我在读书啦。 question: what are you doing? answer: i am reading.

The first thought that came to mind was that you were trolling lol. Like one of those 'You should be studying' trolls. Because that's just it. 读书啦 is subjective. 读书 means to study, or - literally - to read. 啦 is an expressive term. I can't think of an English equivalent but something similar would be like saying "Duh?" It's to that effect. So when you say 读书啦(!), it can just mean "Study!", like when you're telling someone to study. At the same time, it can be a reply. Like if someone asks you what you're doing, you can tell them “我在读书啦＂which is "I'm studying!" Now if you notice that I use exclamation marks to explain, it's because using 啦 kinda makes the term come across that way. It's the same as saying "吃饭啦!" ("Eat!") "好了啦!" ("That's enough!/It's done!")

when i saw it , morning reading comes into my mind .imagine the scene:the bell rings ,then somebody shouts this'读书啦'。it means it is time to reading.:)