After a 40 issue season 8, Joss Whedon and Dark Horse got a little more complex for season 9, expanding to two ongoing series, two mini-series, a one-shot, and an anthology arc.

There are 62 issues worth of material.  25 issues each of Buffy and Angel/Faith, 5 Spike, 5 Willow, the FCBD special, and then the DHP arc (which counts as one issue).  For the most part, each series breaks down into 5 issue chunks.  I’m mainly just alternating between those, although I do swap some of the one-shots around too, to vary things up.

Volume 1
Buffy #1-4
Angel and Faith #1-4
FCBD Special
Buffy #5
Angel and Faith #5
Buffy #6-10
Angel and Faith #6-10
Buffy #11-13
Angel and Faith #11-14
Buffy #14-15
Angel and Faith #15

Volume 2
Spike #1-5
Angel and Faith #16-20
Willow #1-5
Buffy #16-20
Angel and Faith #21-25
Buffy #21-22
Dark Horse Presents #25-27
Buffy #23-25

The main bits of continuity to remember:
-Spike mini goes between Buffy #10 and Angel #18
-Willow mini goes between Angel #14 and Buffy #20
-Dark Horse Presents #25-27 spins out of Buffy #22.
-the Drusilla mini that never came out would come after Angel #9.

Season Ten

This season scaled back slightly on content, in that there were no mini-series, but the Buffy series did run an extra 5 issues.  There are a total of 56 issues worth of content.

Volume 1
Buffy #1-5
Angel and Faith #1-5
Buffy #6-9
Angel and Faith #6-14
Buffy #10-15

Volume 2
Angel and Faith #15
Buffy #16-18
Angel and Faith #16-18
Buffy #19-20
Angel and Faith #19-20
Buffy #21-25
Angel and Faith #21-25
Buffy #26-30
Dark Horse Day Sampler (final story)

The main bit of continuity between the two series was when Angel guest starred in Buffy #16-18.  The 15th issues of both series shared a scene near the end of the issue setting it up, and then Angel appeared in Buffy #16-18 while he was absent from his own series for the same timeframe.

Seasons Eleven and Twelve

Due to the shorter runs of these seasons, it’s easier to combine them into a single volume.  As far as I can tell, Buffy and Angel run independently of each other, so they could be placed in any order.  The Giles mini-series was published after season 11 concluded, but takes place during the season.  I feel it works better as a flashback instead of interrupting the Buffy storyline.  As long as Giles is put after Buffy, the three season 11 series could be configured in almost any order.

Volume 1 of 1
Buffy (season 11) #1-12
Giles #1-4
Angel #1-12
Buffy:  The Reckoning (season 12) #1-4