If you want to start with just one book, The Bentley 240 manual gets my vote. To me it is much better than either Haynes manual, especially it's organization, layout, and indexing. I have both Haynes books but have had no need to refer to them since getting the Bentley about 10 years ago.

It lists for $49.95, but you can get it for $40.95 (with free shipping) by joining the VCOA. The membership is $30/year, which includes 6 issues of the club magazine, "Rolling". The address (include car year & model if joining) is:

VCOA
Box 16
Afton NY, 13730
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Bruce Young,
'93 940-NA (current)
'80 GLE V8 (Sold 5/03)
'83 Turbo 245
'76 244 (lasted 255,000 miles)
73 142 (98K)
'71 144 (track modified--crusher bound)
New 144 from '67 to '78
Used '62 122 from '63 to '67