It depends which army you were in.

I can only really speak for the British Expeditionary Force who operated a rotation system at a number of different levels.

The British operated a rotation policy with a Division in the line having one Brigade in reserve and two Brigades "up" at any given time.

Of these two brigades, two of the four battalions would be "up" with one in the second line assigned to work parties and the fourth battalion at rest.

Drilling down even further, each battalion would only have some of its companies up with the remainder in the second line.

Gordon Corrigan calculated that the actual time at the front for a battalion not engaged in battle was less than nine days a month, it being unusual for a Battalion to be in the trenches for more than five continuous days, and in the actual firing line for more than 2 consecutive days.

Troops in the second line would be assigned to duties like brining up rations and ammunition, repairing or building fortifications, burying cables and so on. This was hard work and as it was mostly within artillery range, was hardly free from risk.

Troops at rest would typically be afforded hot baths, have their uniforms laundered, and set free on the locality to spend their King’s shillings on food (egg and chips being a favourite), beer, and women. The tins of plum and apple jam that was so ubiquitous a feature of their rations being used to seduce fair young maidens.

The rotation system was seriously disrupted in the winter of 1917/1918 when Lloyd-George withheld reinforcements from Haig, meaning that Brigades had to drop from 4 to 3 battalions. The effect on the rotation system was disastrous at a time when all hands were needed to switch from the offensive to the defensive following the collapse of Russia and the impending Germans spring offensive. In the words of one officer, his men had become a collection of “deskilled ditch diggers and cable buriers”

In addition to this program of rotation, entire Divisions could be removed from the line en bloc for rest, retraining, redeployment, or reconstitution.

Specialisation for the infantry only really started after the introduction of the Lewis gun and hand grenades (bombs) in 1915, prior to which, all virtually infantrymen were riflemen apart from the battalion machine gunners.

As Lewis guns and grenades became more ubiquitous, a platoon would have specialised bombing, rifle grenade, and Lewis gun sections. Though by late 1917, all infantry would be trained and equipped as bombers. Indeed, the grenade replaced the rifle as the infantry’s primary weapon by 1917.

For the British sector:

Peter Barton – “The Battlefields of the First World War: The Unseen Panoramas of the Western Front” is practically a trench-by-trench survey of the British sector of the front. (The panoramas are unseen because they were taken by stealth and at great risk to the photographers.) You get large-scale maps as well as the photos, and information on trench construction.

Christopher Duffy “Through German Eyes: The British and the Somme 1916” covers the fine detail of the British attacks and their effect on the opposing army. It's a tough read for a Brit, but it dispels some myths.

If you can find a good military history bookshop you may be able to track down surviving copies of actual officers' handbooks on things like trench design and “schemes”. I can't give titles because my copies are packed up in boxes. But a look through such a bookshop's stock list might turn up these handbooks.

French sector: Peter Walther's “The First World War in Colour” has reproductions of actual colour photos taken during the war. They're mostly towards the rear – very few front-line photos, because the colour process needed a long exposure. Seeing the familiar sights in colour for the first time is quite a revelation.