If you are going to be pre-solo for several/many flights until you have regained currency, I would not suggest buying a book at this stage. If you want to anyway, to cover the basics, there are several. I have not read the later ones, but The Glider Pilot's Manual by Ken Stewart is often mentioned by others. See
BGA Shop - (Powered by CubeCart) for other ideas, and/or the books on
Gliding - afeonline.com, Europe's favourite online Pilot Shop, by .
Download a copy of Laws and Rules from the BGA website.
If you are going to be winch launching, have a good look at “Safe Winch Launching” Feb 2010 edition and other stuff on:
British Gliding Association >> Safety >> Safe Winch Launching
Chris N.