Do people here feel that this is a useful practice or are there better, more efficient ways to improve one's technical skills outside of the daily learning process on the job?
It very much depends on what books you're reading, if they're right for you at your current level and helping you take the next step up. it depends on if you're not just reading but also seeking for ways to engage with what you have read....there's a lot of factors.
Actively doing will always teach more than just reading, but if you read, try out, adapt, try again, then you may find more benefit.
The field of "programming" can be very wide, so technical books sometimes help fill those gaps to give you an head start when you end up having to "learn by doing".
I don't really think its a waste of time. However, in my case, unless I get to apply what I learned or read, I will likely forget it. So my one tip would be to ensure you pick up books in areas a little closer to your personal or work projects.
YMMV