This is quite a philosophical, soft question which can be moved if necessary.
So, basically I started my PhD 9 months ago and have thrown myself into learning more mathematics and found this an enjoyable and rewarding experience. However, I have come to realise how much further I still have to go to reach a point where I could even think about publishing original contributions in the literature given how intensively everything has already been studied and the discoveries already made.
For example, I have just finished a 600 page textbook on graduate level mathematics. Although it took me a while to understand everything in it, I learned from this and enjoyed doing the exercises, but realised by the end that I still basically know nothing and that it is really intended as a springboard to slightly more advanced texts. I picked up another book which starts to delve more into one of the specific aspects in the book and again, it is 500 pages long.
Do I have to read another 500 page book to get a sense of something more specific which I can contribute? At this rate, it will be years and years before I am ever able to publish anything. I only ask as I was a few years older than normal when I started studying mathematics so I feel like I am already playing catch-up slightly and that at this rate I will be an old man by the time I know enough to actually contribute anything.
Edit: Many thanks for the comments and I have taken them on board. I don't regret reading the entire book which I have already read as it is a classic but for now on I will focus on reading papers and trying to solve problems, then work backwards and use books as references if necessary.