Hmm, if we're on to locations of good second hand bookshops (for the needs of "our kind" anyway!)...
LINCOLN:
Oxfam: "Roughly opposite" one of the main entrances to the indoor market, (incedentally a large bookstall is just inside this entrance, and occasionally has Commando/War Picture Library etc for 40p each) and Wilko's. The childrens section with Annuals is under the stairs. Antique books are just beyond the till).
Harlequinn Books: Right at the top of the steepest part of Steep Hill (don't bother with "Reader's Rest" halfway up). Opening times are sporadic (not Wednesdays, i recall), and the woman likes long lunch breaks. BUT when you do get in it's a huge treasure trove, though be prepared to dig. She is very helpful and friendly though and does actually seem to know where everything is. Roughly-defined "Children's" section (though that covers everything from 1860's editions of Tom Brown up to the 2008 Bratz annual) is to the right when you go in. They also do antique newspapers, maps and globes.
"The green shop", er, i forgot the name. When you have ascended Steep Hill there is a courtyard with the Cathedral to one side and Castle to the other, with a road straight ahead (past a white with black beams Tourist Info building that's equally unmissable). Carry on down this road and the shop is on the left, and is green. Or perhaps blue. Either way the huge selection of annuals is just inside the door. Oh and the door is stiff so you may think it's closed when it isn't.
CAMBRIDGE:
Only one i know of that's even worth mentioning. Head to the market place close to Lion Yard (now Grand Arcade). To the right of the town hall (there's usually a hot dog stall shaped like an old tram parked there) is a road that goes back towards the Grand Arcade car park. There's also a church on the right as you go down this road, with an alley to the right of it. Go down this alley, ignore the other bookshop there (nothin' doin. Academic and
really antique books are thier trade) and turn left, then right again. Now you'll see a tiny red shop with some boxes outside, that's the one. It's super-packed and three people will crowd it. The Children's section is upstairs, you have to ask to go up as the stairs are rather treacherous. Then start digging! They have lots of Girl's books if that's your thing, plus HEAPS of W. Howard Baker storypaper reprints (

) on a shelf near the window and a green door. However last time i went the shelf was on a rather alarming lean and creaking... so you may find them in a heap amongst spintered wood instead.
BURY ST EDMONDS
I travel there on the bus, so will run from there.
Where the buses stop, there is a war memorial, "the first illuminated road sign in Britain" (it's rather ornate), and a big theatre or something called Athaenium. With the gardens on the left and the town on the right, walk past these and up to where a (closed) cathedral tower is opposite a road. Go up this road and some distance up it is a white shop on the left. However there's none too much of interest to us, unless you like Rupert. Give it a try though.