Sara,
I do charge a fee. There are different “levels” based on what part of the library a family wishes to use. For access to the shelf books, the fee is $50 per year. The same for FIAR totes. If a family wishes to use the entire library, the fee is $90 per year. Of the libraries I know about, my fees are the quite a bit cheaper…but they will go up some this coming year. Liz and Emily Cottrill have been fussing at me for years that I’m too cheap. And my library has grown so much in the last two years that it’s time. So beginning in August, my fees will be shelf books – $75, FIAR $60 and entire library $125…which is still less expensive than others but more in line. $75 is less that we pay for TV for a month here. I do offer to allow families to pay monthly if they need to. There are those who criticize the fact that I charge a fee and say that I should offer it for free as a “ministry.” There’s a lot I could say about that…
So far I haven’t done the business thing. Last year I was in the red over $9000. (!!!!) This year, though, I don’t have the “big” expenses I’ve had in years past like bookcases so, even though I won’t come close to making a “profit” (not that I ever could nor is it the point) the gap will be closer so I may need to do something more official.
I don’t usually like to talk about the money aspect, but it’s something to realistically think about. If I were only lending books that I buy for my family, it wouldn’t be as much of a consideration. But when we decided to open a library, I had to broaden my horizons and collect books that my boys will probably never be interested in, but some of my library kids love. Often my patrons come in with a list from their curriculum (and I ask them to) and I do my best to get as many of those as I can, as long as they fit the scope of the library. It’s very often that a family will join for $50 and I’ll turn around and order $100 (or $200) worth of books that they need for their studies and hoping it will benefit other families as well.
I haven’t been very creative in raising money to fund the library. Sometimes I sell duplicate books. I don’t actively shop for books to resell although some do. Michelle Miller has an annual “yard sale” to benefit her library each year. The funds just come out of our family’s budget. That may not be something everyone can do and there have been times I’ve made hard decisions. (I went without a winter coat to buy The Chestry Oak…I didn’t freeze, just my coat was VERY old but I hate shopping for clothes so I didn’t mind.) 
You might break this part to your hubby gently…