"On books" is a difficult one to determine for a lot of air forces as they don't give away this sort of information.
Also they do use the word "active" and stored aircraft aren't generally included in rest of list.
In fact it gets very confused:
E.g. Slovakia has 12 "active" MiG-29S (though 4 are stored). There's a further 10 or so airframes that were retired without upgrade to NATO standard but also stored.
Anyway some errors that a quick scan picked up:
Belarus:
- Both Su-24 and Su-27 retired in 2012.
Bulgaria
-6 x Mig-21 – last 3 were retired in December 2012
- 12 x Su-25 (actually there's about 28 on books out of 40 delivered – 1 written off, 1 preserved, 10 to Georgia's TAM for spares) – however correct number last operational was 12.
Chile:
- C.101 is actually advanced trainer. Yet full combat capable F-16B/D is listed as a trainer.
Congo:
- Mirage F1 – type never operated by Congolese AF.
Coratia
– AT802 is a single seat water bomber not transport!
- 10 MiG-21s should be listed (though only about 7 operational).
Denmark
- Order for 48 F-35 not confirmed. Currently there's a competition to replace F-16. Number to be ordered expected to be much less (22-36).
France:
- Rafale order overstated. Current plans are for 225 French combat a/c in total including 175 Rafales (5 lost) and rest in M2000Ds by 2015. Total Rafale order for both Air Force and Navy is expected at 225 airframes.
Iran
- F-6 retired
Italy.
- Eurofighter numbers understated (it's 62 as of 2012).
Ivory Coast
- Alpha Jet not reported operational for some time.
Libya
- At least 3-4 MiG-23s confirmed active,
- At least 3-4 MiG-21s confirmed active (review says 1)
- Only 1 Mirage F1 confirmed active – 1 other of the two flown to Malta was lost in accident(review says 2).
Mali
- Lists 9 MiG-21s. None are operational and most are rotting.
Peru
-Those Su-22s retired nearly a decade ago make an appearance!
Spain
- Only about 8 F1M are active (and they're being retired in a few days)
Thailand
F-5A retired and replaced by JAS-39 Gripen
Turkey:
F-4 numbers overstated. Numbers operational even in 2012 were far lower. By end of year it will be about 50 F-4E Terminator 2020.
Overall it's not too bad but Wikipedia is actually more reliable in some cases.
Personally I prefer aviation enthusiast sites such as scramble.nl