I try to be very cautious with this question. There are at least three different web-development frameworks available in Smalltalk. The most prominent seems to be Seaside but there is also AIDA/Web and Iliad.
They seem to be very similiar, but this impresson may be wrong. I wonder who has tried the different tools and can share the pros/cons of the different packages.
A more concrete question would be, do yo know of let's say any software in the bookkeeing area which has choosen to use either of the three (or other) web development frameworks.
Here's one other question. What about maturity and compatiblity. Which framework can one judge as the "most" stable?
99% of my practical experience is with Iliad, so my comparison is primarily about how I got to choose Iliad instead of the others.
Seaside. Tried it first, it is the most known and the first you stumble in. I found it hard to understand (as always, no matter what you use, when it gets to real implementation you always run into issues that fit badly with the general theory), there was little documentation around and the community seemed to care little about telling n00bs how to grow up. I eventually dropped it to check Aida. You find them here: http://seaside.st/ and on IRC at #seaside
Aida. I really liked it, as Janko is extremely patient and supportive. It is large, though. I decided to use it, when I was to start implementation for our project and it was only by chance that I ran into Iliad a few days before starting. You find Aida here: http://www.aidaweb.si/ and on IRC at #Aida
Iliad. The main reason I chose it was that I had just dropped Squeak and switched to gnu-smalltalk. gst seemed to have a lot of ongoing activity on Iliad, so I reckoned I was probably going to get a better assistance with it. It is much smaller as a concept (though it does absolutely everything) and I had very little trouble in getting started and in tweaking it to what I needed for. It also proved to be well managed, as I went thru 3 releases without ever being in trouble with what I had already deployed. You get answers to questions usually in 12-24 hours max, which is fine for me. You find Iliad here: http://www.iliadproject.org/index on IRC issues are usually channeled on the #gnu-smalltalk channel
What we are doing is a GUI for a PostgreSQL database used to manage a large multimedia dictionary, that in turn produces content (among others, for KDE-edu). I have no notion of booking apps in general, so I cannot answer to your question properly.
Yet, speaking as an analyst, a booking app is mostly a DB with an online GUI, so I see no reason why you could not make it. Much will depend on how easily your db can translate into objects. You could use a RDBMS, as we did, and design tables to map objects, or use an OODBMS. Both solutions have pluses and minuses, none of them is going to have an impact on your choice of framework, I'd say.
My suggestion is that you take a month or so in making experiments and see what better fits you. I don't think there is a tool that fits everyone, we are all different and what is easy for me may be hell for someone else (and the other way round, obviously). So try your car before buying it ;)
I would not invest too much time in studying the general theory. Using a smalltalk framework is going to turn upside down the way you make a web app anyway, and your first contact is going to be full of "false friends" you inherit from previous mindsets. Your best asset is definitely going to be "how quick can I grasp what they tell me", and this is largely an individual thing, depending on you and on who you bump into. So I'd really suggest you get your hands dirty BEFORE reading too much theoretical stuff.