Water-logged books, much like water-logged photos, ARE able to be restored–it's just a very monotonous, difficult, and expensive process.
The only people with the time and resources on their hands to do such a project would be the World Government. Unless they're frozen or the restoration began immediately on all of the books, then the bulk of them would be ruined by mildew.
Really, about the only chance those books have is if the World Government wants to save them.
I think it's possible that Usopp was working on it for some time before they even arrived at Water 7, and he may have been working on it while he was alone on Merry, but he just didn't get a chance to finish it and test it out until later. But maybe someone did help him with it…
Well, he probably did start on it sometime before or after they arrived in Water 7, but it's likely that he finished it on Rocket Man. It could even be that he started Kabuto to use it on Luffy in their duel, but was unable to finish it. The other option was that he built it during the period after leaving Longringlongland, but before they arrived in Water 7.
It's only been a few days since they were in Skypiea after all and he definitely had to have created the Perfect Clima Tact sometime after leaving Skypiea and before they arrived in Water 7.
Remeber that Nami pulled out her regular clima tact before she got on Rocketman, but then pulled out the perfect clima tact after she was on.
The Clima Tact is different though, as it's obviously designed to fold up into a relatively compact form. Perfect Clima Tact wouldn't be as compact, thanks to the dials on it, but it's still not that large when it comes down to it.
These are still basically staves though, which makes them more manage than the super slingshot Kabuto. Rather than breaking into sections, Kabuto's staff might telescope down into a single section that Usopp carries around on his back, which is then connected to the catapult.
It wouldn't be much help if it's too hard to assemble.