If you are studying WWI for GCSE there is really no need for you to know in any detail how particular weapons worked. The course is one in political history rather than military history.
I appreciate that you may have a personal interest in topics such as this but I am afraid the teachers who answer questions here are not experts in military history which is a highly specialised branch of the subject.
May I suggest that you use the name of the weapon(s) you are researching as the search term. In that way you are likely to find military history sites with the type of information you seek.
As for books on the First World War .... there are a great many and which is best rather depends on your particular interests. If it's military history you mean, then one of the best publishers in the UK is 'Osprey Publishing'. Many of their publications are to be found in good bookshops such as Waterstons, Blackwells etc.
Here is the link to the WWI section of their site:
http://www.ospreypub...riod.php/per=44