Observing Handbook and Catalogue of Deep-Sky Objects
Luginbuhl, Skiff, targeted toward people with medium to large sized telescopes, this catalogues goes through each constellation in detail, describing galaxies, clusters and nebulae, A good guide for someone with an 8-12 inch scope or larger. Tabulated form, no pictures and very few illustrations.

Deep-Sky Companions
Stephen James O'Meara. Spread across 3 big hard back volumes The first concentrates on the Messier objects, the second on the Caldwell Catalogue and the third (and biggest) is a "the best of " deepsky catalogue/atlas. Updated with modern knowledge, and great observational accuracy. These books will help you find these objects and then confirm that what you see through a small/medium  telescope is actually right.

The Night Sky Observers Guide
Spread across 2 volumes, this publication is a high detailed account of deep sky objects as seen through large telescopes. The first volume covers "Autumn and Winter constellations", the second volume covers  "Spring and Summer constellations". The authors have amassed 100s of drawings of what the objects look like, a must for deepsky fans with large telescopes.

Planetary Nebula
David Hynes, a small format hardback book specialising in only planetary nebula, it goes through them in detail and is a must for anyone interested in these objects, it is a hard to come by book these days.

Star Clusters
Bret A Archinal, Steve J Hynes, a lengthy hardback catalogue of obvious and not so obvious star clusters, also contains a catalogue of objects in the small and large Magellanic clouds as well as observable objects inside the Andromeda galaxy, not a book for people with small telescopes.

Observing The Caldwell Objects
David Ratledge, paperback, Springer 2000. 109 Patrick Caldwell-Moore objects which cover both hemispheres described page by page. Something to do when you have exhausted all the M objects.

The Abell Planetary Observers Guide
Alvin H Huey, if you desire a serious challenge, then try find objects listed in this book, it is a highly specialised catalogue of all the objects in the Abell catalogue of planetary nebulae, it goes through them in detail and shows a photographic negative and a sketch of how you might see it through a 16-30 inch telescope.

Hicksons Group Observers Guide
Alvin H Huey, a specilised catalogue of extremely faint and distant galaxies, only  suitable for people with scopes greater then 18 inches in aperture.  It contains details, finder charts, photographic images of the object and a sketch of how you might see it through the telescope.

Observing the Arp Peculiar Galaxies
Alvin H Huey, a highly specialised catalogue of every galaxy/quaser in the Arp catalogue, these galaxies have all something in common, they either have a quaser "companion" or unusually bright cores, having said that, this catalogue is suited for observers with extremely large telescopes,18 inches and more. It contains details, finder charts, photographic images of the object and a sketch of how you might see it through the telescope. 

The Herschel Objects
James Mullaney, a good survey of the best of the objects surveyed and discovered by William Herschel ranging from class 1 to 8, if you want to retrace Herschels footsteps across the sky, then this is the book for you. It goes into detail of how he found them, equipment he used and suggest a list of objects he should have found but missed, an interesting read !