There was a series of books put out by Grange Books a couple of years ago (2004) called 'The Aviation Factfile'. The one that is most pertinent to your needs is probably 'Biplanes, Triplanes & Seaplanes'. Not strictly WWI but with plenty of useful info - colour photos, artwork, stats, comparison charts, etc. They were originally going for £20 but I got all of mine (there are at least 6 volumes) on sale in Chapters.
A decent book, if you can dig it up, is 'Color Profiles of World War I Combat Planes' by Giorgio Apostolo (Crescent Books 1974).
Another old-time favourite is 'Aeroplanes and Flyers of the First World War' by Joseph A.Phelan (Grosset & Dunlap 1973). Lots of combat art and colour profiles.
Are you looking for artwork or info? There's the old Harleyfords (a bit obsolete, now, but still useful) and of course, the vast range from Albatros Publications (Windsock, etc.).
Not aircraft as such but, in the same Blandford series mentioned by Mosstrooper, was 'Army Uniforms of World War 1' by Andrew Mollo (1977). There are some nice colour uniform plates for pilots of various nationalities.
Finally, for one of my favourite books on WWI aviation, try to find 'Airmen of World War 1 - Men of the British and Empire air forces in old photographs' by Chaz Bowyer (Arms and Armour Press 1975). An absolutely brilliant collection of beautiful, clear photos of man and machine. The female dispatch rider on page 71 is a real corker! I think I'm in love!

(Actually, just saw a copy on eBay for $25US!)
There are plenty of books out there, either general or specific. Yes, they can get expensive, but the good ones are usually very,
very good! (Grab anything you can by Dr.Martin O'Connor!)