The only book I have specifically on WWI trench warfare, other than Osprey's, is this one.
The usual photos, a couple of sketches, just a general overview...
For an excellent source of late war photography, try to find this one. Superb, clear, large photos of the British, Canadian (yay!), French and German forces involved. Unfortunately, it no longer seems to be available direct from H&C.
Some of the, shall we say, more
mature members of the LAF may remember this (very) little series of booklets from Bellona.
Volume 1 was concerned with
Military Field Works of the 18th & 19th Centuries. Some nice little sketches, well suited to modelling in 3D. If anybody's interested, I could scan some more pages. They cover such things as obstacles, fieldworks, trenches, emplacements, dugouts, mining, bridging, etc.
I've also got a copy of the official 1942
Bilheft Neuzeitlicher Stellungsbau, printed as
German Field Works of World War II which illustrates many of the fortifications prescribed for various situations,
complete with dimensions.