"Lumber core" means plywood. I've been selling these for many years with no problems. The ones we sell use oak-based plywood, superior to pine. The top layer is oak veneer for aesthetic reasons.

Thank you, mogator88.
Would you have any advice on the test weight? The case will be filled solidly with books - it is not going to have nick-nacs here and there. To avoid sag over time, do you think I'll need more than 125 lb. test weight, or will making sure the "lumber core" is not pine take care of the sturdiness issue?

fignut - I happened to buy some old oak bookcases off Craig's List recently and they're incredibly strong. I was actually buying a desk from the guy but he had someone else who was willing to take everything - bookcases included - so I threw in the extra $75 for two tall and one short. I think I got a steal and I don't even like Oak LOL.

They're marked Woodcraft Industries. Here's a link to a website that I believe is the same company.

I see a huge difference between these and particle board bookcases I've owned that eventually sagged. If I were buying new, I'd definitely go with something like I have now.

Here is a link that might be useful: Woodcraft Industries

For shelf strength really the most important thing seems to be keeping the shelf 36" or less. 125 lbs is a lot. That's what a person can weigh. I can't imagine you'd have that much weight in books.

Thanks goldgirl, I've actually seen the Woodcraft bookcases at an unpainted furniture store. They do look sturdy. I was told that there would be a six week wait for delivery. Looking around I saw the lumber core at an office furniture store. They were cheaper and I could have them in a week. The gentleman I spoke with brought up shelf test weights. He wasn't sure just what they measured (and neither am I), but said it was an "industry standard". His bookcases' test weights were 125 lb and he assured me this was all that I needed.
mogator, I don't know if that means the shelf should support 125 lbs or not. But a 3 foot stack of books is pretty heavy - more than I can lift. I only want to buy once - not have to replace in a couple of years.
goldgirl, are your shelves warped at all? The office furniture salesman said that unpainted solid wood shelves would warp more than lumber core.
He seemed credible, but I take everything with a grain of salt and try to find out as much as I can from other sources.
Thanks for the help.

fignut - I'm not sure how old these are since I'm the second owner, but the shelves aren't warped. I've got them loaded with heavy books, but haven't owned them long enough to really judge.

I don't know anything about lumber core, unfortunately, so can't compare. I know 125# sounds like a lot, but books can get heavy.

What's the difference in price?

goldgirl, The Woodcraft looked really nice - very solid - and had nice furniture styling. It was about a hundred dollars more (for a 3'x6' bookcase).
The 125 lb. test lumber core bookcase is just functional looking. I don't know how to say this, but you don't really "see" it - it's not stylish - if you know what I mean.
Previous to seeing this case, I had never heard of lumber core, either. And until mogator88, said it was plywood, I thought it was wood boards covered with veneer.
I couldn't really find any info on test weight comparisons online. 100 is supposed to be "standard" and 200 "extreme load", but is 125 OK for a shelf loaded with books over a number of years? It doesn't help that a lot of manufacturers don't even list test weight numbers.