If you happen to be flying the C150/152 you could do worse than buying 'The Basic Aerobatic Manual' by William K Kershner, ISBN 0-8138-0063-3. Can be found on
amazon.co.uk for some £16 so you don't have to pay inflated pilot's shop prices.
Kershner wrote the Cessna 150 Aerobatic Training Manual, and updated that information in this book. Easy and fun to read (reminder to shout 'clear prop' before cranking the engine with the starter should it stop in flight made me laugh). Data on 21 turn spin is impressive (but I haven't tried it myself yet).
Regards
Gerard