I suspect most people have their own personal caveat to this essay. Mine isn't books (I usually give good books to friends after reading them, and bad books to ebay). My own caveat would be raw materials for building or repairing things (electrical components, nuts and bolts, tools, etc).
Until you find yourself having to move.
When I moved from home to college, I really only had 2 or 3 medium boxes of things i wanted/could take with me. Then from college to my first apartment I also had really accumulated nothing of value, and threw away a lot. When I moved from my first apartment to my second, an unbelievable thing happened. My first job brought with it a mountain of stuff. I really didn't realize it as I always kept that place spotlessly tidy (When I start to think through a piece of code I tend to walk around cleaning, its like a mindless activity that helps my mind work) The big mistake I made was using server boxes i swiped from work to move. Next time (which should be in a few months) I plan on using more smaller boxes.
I recently scanned a bunch of books using gscan2pdf; it makes it relatively easy, and integrates some decent utilities like Tesseract for OCR. (Some fiddling with controls meant that I could start playing a movie and mechanically flip pages and punch the scan button.)
Relatively high-resolution scans like 3-500 DPI mean you can get big PDFs (~200MB for a big anthology was normal), but this isn't much of a concern to me: I can clean them up later if I really need space, and in an era where 1.5tb is 100$, 200MB is nothing (1 or 2 cents of space?).
Even though by weight it's much lighter the book shelves are a real pain in the ass.
I was going to spend some little money on the shipping. Then i found out that the company would pay a ship container and give me hardly any financial support to settle there. So now i'm on full pack rat mode!
...and i can't tell you how nice and easy it was to ship only the some thousand books. Everything else by comparisson is hell. You have to fill some isurance forms on everything. Packaging is a pain. You can't separate a bed into smaller boxes like a shelf of books. Etc
I believe anything is OK (even in large numbers) as long as it is easy for one's mind to treat it as one "item". Books (and probably your raw materials) are uniform. You put the next one with all the others and forget it, so it creates little additional drag on you. Yeah, up to a point when you need to expand storage, but the general rule remains.
Them not taking them wasn't so bad. I was more annoyed by the attitude of the staff who made a big song and dance as if they were doing me a favour by even considering taking free books.
A library accepting free books in good condition seemed like a no brainer to me :|