It's sad, but Unittesting books don't get alot of respect on the market, and it seems to me, when people write them, they don't write them with love. The books I've seen have been riddled with typos and code errors. I can deal with the typos, but errors in code is unacceptable.
Having said that the two books that deal with Cactus that I've used as references are:
The latter is pretty good for Cactus. I think you'll find both of these books give you some good ideas, but might not give you the depth you're looking for. But then again, a good book shows you how to use the tools - it's up to you to decide what it is you will craft from them.
Having been one of the proofreaders of both "Junit Recipes" and the german version of "Unit Testing in Java: How Tests Drive the Code" I can assure you that there is a lot of love in them. I'd assume that the same is true for the other books.
Both "JUnit Recipes" and "JUnit in Action" contain tips about Cactus and I think also HttpUnit.
As an aside, I'd rather use HtmlUnit or JWebUnit than HttpUnit - both have a more friendly API.
The soul is dyed the color of its thoughts. Think only on those things that are in line with your principles and can bear the light of day. The content of your character is your choice. Day by day, what you do is who you become. Your integrity is your destiny - it is the light that guides your way. - Heraclitus