Tell your husband that $100 in the first two months is wayyyyyy less than a lot of people have done.

Most people "freak out" in the beginning with how easy it is and the excitement of the new toy and all that (been there done that, still doing it 9 months later LOL) and have spent much more, and then start to slow down and be more careful of the budget.
As someone else mentioned, one way to stay on budget once you decide what the amount will be, is to buy an Amazon gift certificate for yourself at the beginning of the month. Once the GC is gone, no more buying books until you get the new GC the next month, even if you spend it all by the 25th or 15th or 5th. Assuming you can stick to that kind of restriction and not keep buying anyway.
You can also supplement that during the month, or however often you can, with turning coins to "cash" at a Coinstar machine near you that gives Amazon GCs, if you have one. Lots of folks do that, too. Make sure you keep an eye out in the Book Corner forum to the Free Books thread and the Bargains thread, as a way to make the book budget go farther. Amazing how many freebies (not the old classics, but "current" books) there are in a month, as well as good bargains!