Welcome to the forum!
Not sure about the best book - I'll have a think to see if I can make any suggestions.
In the mean time, I might be able to shed some light on a few of the phrases mentioned in your post.
Bunker first: The short answer is a steam loco running backwards ie. with it's bunker leading (as opposed to running chimney first ie. forwards).
The term bunker first would specifically refer to a tank engine which has a coal bunker behind the cab, as a tender engine would be running 'tender first' if running backwards.
Operating steam locos 'bunker first' (or tender first) was common practice if there were no turning facilities to turn the loco to run forwards and this is now commonly seen on preserved railways. Running backwards is not a problem at all for the loco itself, but running forwards is always preferable for the crew as they are then facing all of their controls as well as the direction of travel.
Headshunt: In short, it is a siding used to access more sidings. It can also be a siding used to carry out shunting moves without having to access the main running lines. A diagram here would help enormously, but I can't manage that in this format!
Down Starter: It is a signal, and you are correct that the 'Down' refers to the direction of travel. In general terms, railways use 'Up' to refer to the direction of travel towards the main terminus of the line - almost always running towards London in the UK. 'Down' is the opposite direction ie. running away from the main terminus.
A Starting signal (or Starter) is the signal at the end of a station platform which signals trains away from the platform. Each platform will have it's own Starting signal. See these at my local station:
http://www.railwayforum.net/gallery/...searchid=10060
NB. Not to be confused with an 'Advance Starter'!
Headcodes were displayed by different combinations of lamps (or discs on the Southern Railway) - the pattern displayed represented a different class of train, or a specific route for the train. I don't know specifically what a 'Class B Headcode' is, but I'm sure another member of the forum could enlighten you.
Likewise, I'm not sure of the specifics of a 'Trap Point'. The terms 'Trap Point' & 'Catch Point' are often mixed up, but refer to different types of points which protect main running lines by either derailing or stopping a train that may be about to enter the main lines without permission. I think the differences between Trap & Catch point have been discussed on this forum before, but I'm not entirely sure which is which!
The enthusiast section of the UK & Ireland Heritage Railways website does have some useful information such as steam locomotive and boiler diagrams, signalling information and a railway glossary, which probably gives better explanations than I have here! Follow the link here:
http://www.heritage-railways.com/enthus.php
Anyway, hope this helps. Please ask any other questions that you may have - there's some very knowledgable folk on this forum!
Regards,
Tony