Hi Huw 

We have a lesson on the "se aspectual" which is an emphatic use of the reflexive pronoun. Here's our lesson. If you mean it's used to say that you are doing that action right at the moment of speaking, I'm not quite sure that "me" is implying that itself. When you use the present progressive (estar + gerundio) you are already talking about actions that are happening at that moment. I'd say the use of "me" is emphasising the involvement of the person in doing that reading, as we explain in the lesson.

Saludos

Inma

 

Hola Huw

Yes, if you want to add that extra nuance of "involvement or completion of the action" from the subject you may then use the sentence with "se":  "Javier se leyó el libro que le di." But you can still use it without the "se" and be correct. The use of the reflexive pronoun is just adding that extra nuance. 

It's the same with your initial sentence "Me estoy leyendo este libro." - You can also say "Estoy leyendo este libro", both correct with the same meaning although the use of the pronoun reflects the more intense involvement in the reading by the person.