Chewing Safe woods for wooden cages

A billy bookcase is okay as long as it is laid down flat on its back with all the shelves removed. If you did that it would give them an internal space a little smaller than 202 cm long by 80 cm wide, giving them a little less than 1.6 square metres of living space, with 28 cm tall sides. However you would need to do something to extend the sides as 28cm is too short and guinea pigs will jump out.

Edit to add: Alternatively you could get several of the $8 lack coffee tables https://www.ikea.com/au/en/catalog/products/90161673/#/00193664
If configured in a 2 table wide x 3 tables long rectangle that would give you 165 cm length and 110 cm width, for at total of 1.8 square metres of living space. Alternatively 2 tables by 2 tables would give you 1.2 square metres of space which is the ideal size for a pair of guinea pigs of either gender. I have seen guinea pig cages made of lack coffee tables, with plexiglass/clear acrylic sides.