There are definitely a few books I keep going back to:
Chamberlain & Ellis - British and American Tanks of WW2. This was my first armour reference book (given to me 40 years ago), and it's still amazingly comprehensive and reliable. I think some entries could do with revising (e.g. the Sherman), but it holds up well. Everyone should have it.
Hunnicutt - Sherman. Still the definitive Sherman book.
Jentz - Panzer Truppen I and II. Another must-have.
Zaloga - Armoured Thunderbolt. The service history of the Sherman, and an attempt to correct the myths. An excellent companion to Hunnicutt.
Restayn - Tiger I on the Eastern/Western Front. While more concise than others, this seems to be the best reference on the Tiger I's service history.
Fortin - British Tanks in Normandy. Despite some dodgy English and not enough photos, it's another definitive work.
Bernage - The Panzers and the Battle of Normandy. Really comprehensive unit-by-unit account of the German armoured forces in Normandy. An excellent companion to Fortin.
Oliver & Starmer - The New Breed. A great book on British armour at the time of Alamein.
Restayn and Miller - The 10th Panzer Division. One of the best of the JJ Fedorowicz coffee table books. Lots of really good photos of this unit, both before and during the Tunisian campaign.
Concord - they are great sources for photos but the accuracy varies. I really like Zaloga's titles (mostly US and Soviet subjects), and the titles devoted to British armour (Fletcher writes good captions). The German titles are less reliable, and anything by Michulec is suspect.
Osprey New Vanguard. I buy these because of the broad range of subjects they cover and because they are cheap and don't take up too much space. Again, Zaloga dominates this range, because he's delved into subjects like French and Eastern European armour. He's also not afraid to challenge the perceived wisdom on German armour. Fletcher's British armour titles are well written and honest.
Squadron/Signal. I prefer the older titles. They had well chosen photos and useful technical drawings.
Tankograd - Wehrmacht Special series. I have a few of these; the info is limited (and the captions aren't great) but they have a lot of photos which haven't been published elsewhere.
Tankograd - Technical Manual series. The best source I've found for details that aren't available elsewhere. I only have a few, however.
Some books are over-rated. I find Spielberger's books quite badly organised, and the translation seems off in places. Panzer Colours used to be indispensable, but it's been shown to have a lot of mistakes.