I already have a personal archive that is probably larger than the library of Alexandria. If this kind of stuff matters to you get a hard drive and start your own today. Once it gets large enough, put it on a NAS with RAID5 and a hot spare.
RAID is for data availability, not backup.
If this is all new to you, look at FreeNAS for a "turnkey" solution. As well as being a solid piece of software, their guides can help you configure the necessary hardware.
The amount of RAID 5 arrays I've seen die during the rebuild process (because one of the other drives, with an equal level of wear of the failed drive, failed due to the increased IO) is non-zero.
That said, we still should think about fire-resistant public libraries, not just personal collections.
Not good enough. RAID6 is the absolute minimum, as with current disk sizes it's too easy to lose the whole array due to an additional disk failure while one disk is being replaced.