Based on the Marvel Previews #66, February for April 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 #590
Amazing Spider-Man #590, Wolverine art appreciation variant
Amazing Spider-Man #591
Amazing Spider-Man #592
Amazing Spider-Man #592, Wolverine art appreciation variant
Amazing Spider-Man Family #5
Marvel Adventures Avengers #35
Marvel Adventures Spider-Man #50
Marvel Apes: Amazing Spider-Monkey #1
New Avengers #52
Ultimate Spider-Man #133
Guest Apps
Avengers / Invaders #10
Avengers / Invaders #10, Mitch Breitweiser variant cover
Marvel 70th Anniversary Celebration
Marvel Assistant-Sized Spectacular #1
Marvel Assistant-Sized Spectacular #2
Marvels: Eye Of The Camera #5
Marvels: Eye Of The Camera #5, B&W variant
Official Index To The Marvel Universe #4
Timestorm: 2009-2099 #1
Reprints
Marvel Apes HC
Marvel Triple Action #1
Marvel Zombies 2 TPB
Secret Invasion Chronicles #2
Spider-Man 2099 Vol. 1 TPB
Spider-Man: Brand New Day #7
Spider-Man: Death And Dating Premiere HC
Spider-Man: Kraven's First Hunt TPB
Spider-Man: Spider-Women Digest TPB
Spider-Man: Torment Premiere HC
Spider-Man: Torment Premiere HC, variant edition
Ultimate Spider-Man Ultimate Collection Book 2 TPB
What If? Secret Wars TPB