The hint that the snakes had to be taken away from some clues was in the clue to 24 down. I have not been able to see a hint how to solve the clues generally but it might be evident to those who are more clever than I.
I am sorry but I cannot recommend a book to help you with crosswords such as these - but could I suggest you ask the question to others within the main Q&P category - it will be seen by more people that way. You might like to copy my link to this particular crossword so others know the type of crossword you are enquiring about.
Finally, have you completed it now or can I help with any further clues? Best wishes, Catron.