Haynes manuals for the Mk.4 Golf cover almost all of the engines.
I've shamelessly stolen most of this from
mattytdi on the uk-mkivs board:
The Mk.1 Leon is based on the Mk.4 Golf platform, the A4 platform shared with the Mk.2 Toledo, Audi A3 and TT, Bora and Skoda Octavia.
GREEN Book Haynes Number 3727 - April 1998 - 2000 (R to X registration) 4-cyl Petrol & Diesel
Petrol
1.4 DOHC (55kW) - AHW, AKQ, APE, AXP
1.6 SOHC (74kW) - AEH, AKL, APF
1.6 DOHC (77kW) - ATN, AUS
1.8 DOHC (92kW) - AGN
1.8T DOHC (110kW) - AGU, AQA, ARZ
2.0 SOHC (85kW) - APK, AQY
Diesel
1.9 (50kW) - AGP AQM
1.9T (66kW) - AGR, ALH
1.9T (81kW) - AHF, ASV
Does NOT cover 4-Motion (4x4) or Cabriolet
Does NOT cover 2.3 litre V5 engine
Does NOT cover 1.9 litre 'PD' engine
BLUE Book Haynes Number 4169 - 2001 - 2003 (X to 53 registration) 4-cyl Petrol & Diesel
Petrol
1.4 DOHC (55kW) - AHW, AXP, BCA
1.6 SOHC (74kW) - AEH, AKL
1.6 SOHC (75kW) - AVU, BFQ
1.6 DOHC (77kW) - AZD, BCB
1.8T DOHC (110kW) - AUM
1.8T DOHC (132kW) - AUQ
2.0 SOHC (85kW) - APK, AQY, AZU, AZJ
Diesel
1.9 (50kW) - AGP, AQM
1.9T (66kW) - AGR, ALH
1.9T (74kW) - ATD, AXR
1.9T (81kW) - AHF, ASV
1.9T (85kW) - AJM, AUY
1.9T (96kW) - ASZ
1.9T (110kW) - ARL
Does NOT cover 2.3 litre V5, or 2.8 or 3.2 litre V6 petrol engines.
Does NOT cover V5, 4-Motion, R32 or Cabriolet models, or new Golf range introduced January 2004
Everything in these manuals is relevant apart from some of the bodywork and interior details.
I'm not sure what you can do if you've got a V5 though.