I recently stumbled across a very useful site on which you can improve all aspect of the dreaded subject of music theory, whilst having a bit of fun:
Learning to read music can be quite a struggle for beginners, but on this site there are note reading exercises which you can tailor to your own needs. So if you are an aboslute beginner, and only want to practice natural notes in treble clef, from middle C to the G above, then you can set the site to just test you on those notes. If you fancy testing yourself on bass and treble in all keys, or even the more obscure tenor clef, then you can do that, too.
All the way a score is kept, and you get the percentage of correct answers displayed. As someone who teaches, I find this works particularly well for teenage boys, as rather than having to look at tufty old scores, they get to go online, or better still, download an app, and can click on buttons and see a score at the end of it – much more fun! There are quite a number of other exercises you can test your skills on, from key signature, interval and fretboard identification to ear training exercises.
There are also lessons on the basics of music, which are pretty handy (clefs, notes, scales, etc.), to some slightly more obscure tools, such as for building 12-tone rows. You never know, someone might be writing a 12-tone opus on the train next to you.
Most of the time when someone is pushing a site on a blog, it is because of a (monetary) kick-back, but I can assure you that in this case, we have no relationship with this site, we just found it damn useful!