The thing people aren't getting about the library
With the call to build it somewhere pretty or where they can build it bigger is:
The people who most need and use the library currently are quite likely to get there on the bus. Yes, the bus you think nobody uses; although if you stand outside the current library and look at those buses go by, you'll quickly be disabused of that particular brand of suburban idiocy.
The current library works well because it's on one of the two most heavily bus-travelled corridors downtown (Guadalupe). A location on Cesar Chavez too far from Congress, on the other hand, won't be an easy trip for many of the current patrons.
Look at the map (zoom in on the lower-right inset). Notice how many buses go right next to the thing. Most of the rest of the buses are three blocks away on Congress. So, a huge chunk of routes don't require any walk at all, and most of the rest require a 3-block walk at most.
Now, consider the proposed new site at what's now the water treatment plant. Going by current routes, two come fairly close, but the big conglomeration coming down Guadalupe/Lavaca will be about two blocks away; and the Congress routes about five blocks away.
This doesn't sound like much to walk, and it wouldn't be for most of us. However, as somebody who hasn't been able to walk well for quite a while now and used to serve on a commission where we were often taking up issues important to those who are mobility-impaired, I have more appreciation than most for what a pain in the ass this is going to be. Oh, and don't forget, unlike most of the people involved with this decision, I've been to this library many times - and I can tell you that at any given time, a huge number, possibly even the majority of the patrons arrived on the bus, and a large fraction of those are either elderly or in wheelchairs or both. For THOSE people, two more blocks is a lot to ask.
Don't move somewhere which makes the library less accessible to those who need it most just for the sake of being pretty. Please say no to moving the central library off the main bus lines.
Update: Several commenters have commented along these lines (paraphrased, with my response):
"Isn't commuter rail going to a transit hub at Seaholm anyways?" - please do yourself a favor and read this category archive and start with this post, OK? Short summary: It ain't going to Seaholm for decades, if then. And Seaholm is still a couple-blocks'-walk from this site.
The buses will just be moved to go by the library - this isn't going to happen either, folks. Long-haul bus routes don't make two-block jogs just for the hell of it (people already complain about how supposedly indirect these things are). Each one of those bus routes might deliver a dozen passengers a day to the existing library - enough to make it a valuable part of the demand for the current route, but not enough to justify hauling a long, heavy, bus around a bunch of tight corners.