If your book collection has expanded to the point where you've decided to put it in a storage unit, you might find these tips helpful.
Buy a few cheap bookcases for the unit
Even though it might be easier to simply stack the books on the floor of the unit or leave them in boxes inside it, it's best to purchase some cheap bookcases for this storage space and place your books on these, instead. If you're keeping these books in storage, rather than donating or selling them, then you probably want to read them again at some point or lend them to friends. If this is the case, you'll find it much easier to pick up the books you want during your visits to the unit if they're on bookcases.
If, for example, you leave your books stacked on top of one another on the floor, and you want to get a book that's at the bottom of the stack, it will be hard for you to do this without the entire stack collapsing. Additionally, if you ever bring a drink with you into the unit and spill it, any books that are on the floor or in cardboard boxes near the spillage could get ruined.
As an aside, these bookcases will also make the unit look like a library or bookshop, instead of just a room filled with boxes, which will make perusing it much nicer.
Use an organisation system when arranging the books
If your collection of books is large enough for you to require a unit to store it in, then it would probably be a good idea to use an organisation system when arranging it on the bookcases. You could, for instance, arrange them alphabetically, or according to their cover colour or genre.
This will make visiting the unit and selecting a book to read a pleasure, rather than an annoyance, as you won't have to scour each shelf looking for a particular book but will instead be able to go to the correct bookcase and shelf and find it right away. Furthermore, if you eventually move to a property that has enough space to store this collection, you can pack the books into boxes according to whichever system you've chosen and will then be able to arrange them much more quickly when you put them on the bookcases in your home.
Contact a company that rents storage units to learn more.