Derek Robinson - His three WW1 RFC and two WW2 RAF air combat novels are great. He also did one involving early war bombers - haven't read that. Piece of Cake and Goshawk Squadron are perhaps the most well known.
Len Deighton - if you like spy novels, he is the daddy. Forget LeCarre et al, his Samson series is superb. He also wrote The Ipcress File (the film with Michael Caine as Harry Palmer). His Bomber novel is excellent, describing the last flight of a Lancaster in 1943.
Willi Heindrich (?) - wrote Cross of Iron. The book is much better than the film with Coburn in it. You really get a feeling for being on the wrong side of the Eastern Front (if there was a right side).
Bernard Cornwell - the Sharpe series is really good stuff, if a little formulaic. His descriptions of battle are definitely the best I've read of any conflict - he really knows how to sustain the pace of the action over many pages. Sharpe's Waterloo and Sharpe's Rifles are two of my favourites. There's 19 books in the series, IIRC - read them all but can't remember exactly.
Patrick O'Brian - creator of the Jack Aubrey/Stephen Maturin series of book set in the Napoleonic Royal Navy. Master and Commander the film is based on two of the books. Just started reading them and am really enjoying it.
C S Forester - creator of Horatio Hornblower. As detailed as Parick O'Brian in terms of historical accuracy. You really get a feeling for how difficult handling a tall-ship was and how tough life on board was. Hornblower is nothing like the TV series character. He's much less likeable; a pretty harsh character in some ways.
Joseph Heller - if you haven't read Catch-22 you really should check it out. A long book, but the kind novel you'll read in a couple of days because it is simply so engaging. You'll probably read it again straight afterwards. The sequel, Closing Time, is dissappointing - but that was to be expected. His other books are interesting, but Catch-22 is his masterpiece.