In reply to Ben Watts:
South Devon and Dartmoor is the best guide, ever. Oh, you mean the best guidebook for Dartmoor? Conveniently that is also the South Devon and Dartmoor :P
That aside, since access to the remaining copies of this hallowed tome is now difficult to acquire...
Rockfax West Country Climbs is useful, if only because it has prettier pictures than many, but also has pretty good coverage of the smaller crags such as Sheepstor.
In terms of freely available online stuff:
If going to Dewerstone, print off the PDF topo from rustypeg/javu (no pictures).
Javu has the best stuff for bouldering, but it also has a good number of topos (including photos) for places such as Haytor/Low Man and is well worth a look.
South Devon Mountaineering Club also has topos for a few of the crags (e.g. Sheepstor).