Hi MW,
You don't need to download an app, this is just an option for you.Edited: You do need to download an app, one for each device you want to use for the book.
(For some reason when I see "app" I think only of mobile, sorry about that.)
You can view the book on your computer, or having already paid once for the book, you can view the same book on your mobile.
You don't need to worry about the space consumed by a digital chess book. Although I can't say precisely the size of any digital chess book, it will on the order of 1 MB. To be conservative, say it is no more than 2 MB. Then 512 books would be 1 GB, and it's easy to see your 128 GB drive will not fill up with
chess books any time soon.
That said, as a non-computer person you still need to worry a little about your SSD drive.
1. You can lose it. Your computer might be lost or stolen, or damaged by fire, flood, accident (spilling coffee on a laptop is surprisingly common), etc. These days a ransomware-attack is the most likely way to "lose" a hard drive.
2. It can fail. An SSD is quite hardy compared to a spinning disk, nevertheless there is still a small probability that it might fail.
3. It can fill up. 128 GB seems like an unimaginably big drive, but consider. I assume you are using Windows, so 20 GB will be taken by Windows as a Recovery Partition, therefore unavailable to you. Then the Windows operating system itself will need about 40 GB. That's half your drive right there, before you have added even one user file! Further, for efficient operation a hard drive needs about 50% free space! (Something to think about when shopping for your next computer.)
Edited: Thinking about cloud storage made me think about another danger.
4. You might erase it. The classic one is the UNIX typo "$ rm / *" instead of "$ rm /*". I didn't do that one, but I did delete a project once when I answered wrongly at the "Are you sure?" prompt. (I was very tired.) Another time I dragged my network folder (mouse-slip) into someone else's network folder, but due to wrong permissions I could not move it back! Nor could I even look at it! It was said about chess, but applies to computers just as well: The mistakes are all there, waiting to be made.
It's particularly relevant because of the way cloud storage works. If your only copy is in the cloud, deleting a file means you may never get it back. If your cloud folder is synced automatically, deleting the local file also deletes it from the cloud. Some cloud providers also provide a backup, but doesn't that beg the question?
Your answer to all three (edit: now four) above problems is the same. You need to backup your hard drive. The method I use is a dead-simple one recommended in PC Magazine ages ago. Buy an external USB hard drive, create a backup folder on the external drive (e.g. backup_users_20201225) and drag your Users folder from the PC into the backup folder on the external drive. As for which drive, anything in the 1 TB to 5 TB size will be more than enough for now. If you outgrow it then just buy another one. Having more than one backup drive is also a good idea!