Based on the Marvel Previews #65, March for May 2009, I'd say you need to pre-order the following books in order to keep up with Spider-Man.
Core Titles
Amazing Spider-Man #593
Amazing Spider-Man #594
Amazing Spider-Man #595
Amazing Spider-Man #595, variant cover Adi Granov
Amazing Spider-Man Family #6
Marvel Adventures Avengers #36
Marvel Adventures Spider-Man #51
New Avengers #53
New Avengers #53, variant cover Chris Bachalo
Spider-Man: The Short Halloween
Ultimatum: Spider-Man Requiem #1
Guest Apps
Agents Of Atlas #5
Avengers / Invaders #11
Avengers / Invaders #11, variant cover Mario Alberti
Marvel Apes: Grunt Line Special
Marvel Reading Chronology
Official Index To The Marvel Universe #5
Timestorm: 2009-2099 #2
Ultimatum #5, fallen heroes variant Olivier Coipel
Ultimatum #5, gatefold variant David Finch
Ultimatum #5, regular cover David Finch
Ultimatum #5, sketch variant Ed McGuinness
Ultimatum #5, variant cover Ed McGuinness
Reprints
Black Widow: The Sting Of The Widow HC
Black Widow: The Sting Of The Widow HC, variant edition
Essential Spider-Man Volume 9 TPB
Marvel Masterworks: The Amazing Spider-Man Vol. 11 HC
Marvel Masterworks: The Amazing Spider-Man Vol. 11 HC, variant edition
Marvel Masterworks: The Amazing Spider-Man Vol. 2 TPB
Marvel Masterworks: The Amazing Spider-Man Vol. 2 TPB, variant edition
Spider-Man J Vol.2 Japanese Daze Digest TPB
Spider-Man Magazine #6
Spider-Man: Family Ties Premiere HC
Spider-Man: Noir Premiere HC
X-Men: Worlds Apart HC