Getting the book is the best first step. It is a mine of good advice and information.
Have a look here in the Vowell Art section of the Gallery for exploded diagrams of the Ruckstells:
app.php/gallery/album/249 This will help familarise yourself with the bits and how they work, and will explain my comments below.
From a safety point of view you want to be sure the Ruckstell changes smoothly without any slop, so check each link in the shift levers and rods from one end to the other and the shiftlock mechanism internals. A 'sloppy' change is not good and must be repaired.
Next, depending on the shift lock (change mechanism - see the diagrams) there are parts that are prone to wear:
In earlier 'snub' changers the 'yoke' part P122 can have the square hole rounded (not that common) while the more likely wear is the tapered plunger end (part P117) which becomes rounded and does not hold properly in the P113 slider. This is a BIG deal. Also the spring P163 can lose strength and should be replaced. Chaffins can supply new ones.
On later 'long nose' ones the tapered end on the 'new' P122 'yoke' and/or the taper on the plunger P117B gets worn down. They need to have a good taper and the length needs to be checked, as they can get shortened by use (sorry I don't have measurements its a case of fit and check or just replace). Also the spring P163B also needs to be free and strong.
On the 'long nose' broken springs can mean the yoke is not firmly held in place and can move, only inspection will tell.
A cause of sloppy movement can be part P123 and/or the lever P98 getting rounded out on the square hole or by wear in the locking screw. Repairs needed depends on what you may find.
Should these parts be too worn the Ruckstell can get caught mid shift = no drive = no brakes either (this has happened to several of us here - can be hair raising!). Or (a common occurrence - more so than is healthy for drivers) the Ruckstell will simply pop out of gear into neutral under heavy load. Not good.
Some will say that Ruckstells don't have a neutral - that is true IF all the parts are in good working order and properly adjusted. If not, well lets just say that wear does allow the sliding gear (P146) to miss meshing with any gear = an unwelcome neutral.
If possible a change to a later long nose shift lock is a good idea, they have a better positive change throw.
It is a good idea if you suspect an issue or just want to check, to remove the shift lock from the axle and look it over. In a vice to hold it, the 'long nose' change should 'snap' and require a long handle to move it - not by hand alone. If by hand then you have a 'taper' problem! (this test won't work for a 'snub nose' as it doesn't have internal springs to hold it).
CLEAN - Perhaps a real issue if the car has been sitting for a while is that the plungers and springs get gummed up and don't move properly. A strip and clean of the shift lock may be all that is required!
ADJUST a contributing factor can be adjustment of the plunger in the 'long nose' shift lock. A simple adjustment to increase the spring pressure on the plunger is easy to do and may be all that is required.
There is nothing better than a strip down and check of all bearings and parts for wear.
If the strip, check and rebuild work sounds threatening then wait for the book. It will lead you through the process step by step and explains as its goes.