Fun parts of the Gatekeeper trilogy: Buffy, Angel and Oz go on a European vacation by skipping school and dealing with Angel's sunlight/secretive feeding issues.

It's an interesting group of characters to have interacting, especially isolated out of the country. Only supernatural beings can travel the Ghost Roads, so that makes that trio the ones who have to go. Angel seeing Jenny in the Ghost Roads (where all the lost souls with unfinished business go who wonder around aimlessly and attack the group while trying to escape) and wondering if that's where his soul goes when he's not ensouled is probably the most interesting character moment in the trilogy. It also explores just what exactly happens to souls with unfinished business in the Jossverse (it's such a horrific concept that I personally accept it as my headcanon in lieu of anything to contradict it).
The Buffy/Angel books are gold if you're like me and prefer the high school years + Angel investigations. It's where to go if you want to read, say, Angel actually in-character! The majority of the books, well after the era ended, continued to heavily feature the early seasons (season 3 is so hilariously cramped) and the crossovers are very kind to the Angel Investigations team (arguably even featuring them more, given there's a lot of the Scoobies coming to L.A.). The crossovers also allowed characters who never met on screen to interact (in case you ever wanted to see Tara meet Fred).
It's really quite shocking how the non-canon writers during the show's run actually wrote everyone in-character when they didn't have carte blanche to do anything they wanted and were confined to fitting things into the shows' continuity. And they actually bothered with plotting the fantasy/supernatural/horror side of the story, along with that more confined character development (character moments). The current comics completely suck with that.
The books sometimes went a little bit big (like, Sunnydale would have to remember that!) and the budget would never have worked on television sometimes, but it was still in the style of the show tonally. There are an awful lot of fairs and carnivals coming to Sunnydale. That's always amusing.
Some of those books have some really great material like in Monster Island where Buffy thinks about bad father figures (the book deals with Doyle's demon father) and reveals that Angel did discuss his father issues with her, which she compares to her situation with Hank. It's one of those moments you just wish had been in the show, because it's something these characters had in common so much. And the atmosphere in Child of the Hunt makes it one of my favorites. It features another Slayer who was once in a strange, unorthodox relationship and was also a mother (Lucy Hanover, who originated in the early WB adverts, with her homunculus son, Roland, with the Erl King). There are also sometimes these amazing little moments like Angel having beer with Giles, hoping he can taste it, but he can't, Angel protecting Buffy from looking as the Wild Hunt goes by, telling her she can't look (which if you look at them, you'll be taken away) and Cordy sneaking into Buffy's room to get supplies and running into Joyce. How I Spent My Summer Vacation redeemed Buffy's bad behavior in When She Was Bad by making it actually make sense (explaining why Buffy was so haunted by the Master all summer), as well as building up Angel, Jenny and Giles' friendship (they protected Sunnydale in Buffy's absence) before the Angelus fallout. It indeed even explains why Angelus already had an invite to Giles' house.
What I've read:
1998-01 - Buffy - Coyote Moon
1998-08 - Buffy - Blooded
1998-10 - Buffy - Child Of The Hunt
1998-12 - Buffy - Return To Chaos
1999-01 - Buffy - Gatekeeper 1 - Out Of The Madhouse
1999-03 - Buffy - Gatekeeper 2 - Ghost Roads
1999-05 - Buffy - Gatekeeper 3 - Sons Of Entropy
1999-11 - Buffy - Sins Of The Father
2000-04 - Angel - Not Forgotten
2000-08 - Buffy - How I Survived My Summer Vacation
2001-05 - Buffy & Angel - Unseen 1 - The Burning
2001-06 - Buffy & Angel - Unseen 2 - Door To Alternity
2001-08 - Buffy & Angel - Unseen 3 - Long Way Home
2001-08 - Buffy - The Lost Slayer 1 - Prophecies
2001-09 - Buffy - The Lost Slayer 2 - Dark Times
2001-10 - Buffy - The Lost Slayer 3 - King Of The Dead
2001-11 - Buffy - The Lost Slayer 4 - Original Sins
2002-12 - Angel - The Longest Night
2003-03 - Buffy & Angel - Monster Island
2003-04 - Angel - Sanctuary
2006-01 - Buffy - After Image
2006-04 - Buffy - Carnival Of Souls
2006-10 - Buffy - Portal Through Time