Inventory System Design for Rental Company

I've been building a system for a client who sets up the AV systems for hotel banquet halls and event rooms.  They've got about 100 offices that handle about 150 hotels.  Each hotel has about 5 rooms that are booked an average of 5 times a week.  For each event they have a team pull LCD projectors, cables, speakers microphones, screens, etc... out of their local office inventory.  Anything they don't have they pull from one of the other offices or a shared regional warehouse.  
sound fun yet?
So, right now I've got it set up so the user logs in, books and event, adds an item to the order, adding it automatically locks the number of items in stock for the event - individually.  So, if they need 2 projectors, they get projector serialnumABC, serialnum123, leaving serialnumXYZ available.  Now, if they need more than they have, it goes to a "hot sheet" which shows up on the front page of all the warehouse managers' system (of those warehouss associated with that office).  The warehouse manger has to allocate their products to the office for the order and produce a pullsheet for that office for that day, and lock down those inventory items for the period of time that the borrowing office needs them for....  ok.... brain freeze.

Now, I've done all this in pieces.  Basically, the client started with 1 office and no real inventory issue since they only really had a few things of value.  Next, 5 more opened, so they kept track of the items that were shared in their heads.  Then about 10 more... the manager started keeping an excel sheet and I simply pulled it into the system.  then they purchased another company and dropped 120 more *$&#@^@# offices on my system.  Now I've had to add all sorts of crap to their system and I feel like I'm wading through piles of duct-tape and coathangers everytime I need to make a change.
Where can I find REAL information about how this sort of system is SUPPOSED to be made.  What I've got is simply insane and, if I get hit by a bus, my assistant (or another developer) is never going to figure it out.  Any link to seriously detailed literature or diagrams would be helpful.